SilverMax_04

I’ve been planning this mileage test for over 6 months. I though it would give me definitive information on whether premium gasoline gives better highway mileage than regular gasoline (in this case, actually the cheapest gasoline available – which in Iowa and eastern Nebraska is 10% ethanol blended 89 mid-grade). Looking at the results, I don’t think they are definitive. I expected that premium would give somewhat better highway mileage than the cheapest gasoline, but the results show the mileage was slightly worse using premium. I actually think other factors beyond my control helped produce these unexpected results. I’ll discuss these results in more detail after reporting the test basis and the test results.

The plan was to drive East from my home in Colorado Springs to west suburban Chicago burning the cheapest gasoline grade available. Since I burn only 85 octane regular gasoline when home, I was starting out with a tank full of this cheapest grade. (See post #6 on the thread below for an explanation of why 85 octane gasoline at 6 K feet elevation acts like 91 octane at sea level):
http://forums.maxima.org/showthread.php?t=396716

On the trip West I would burn only 91 octane premium. I essentially followed this plan, except I started buying premium just before I got to the Chicago area – only 4 gallons to keep from running out before I got to Iowa on my trip back. (The price of gasoline in Iowa is cheaper than Illinois because of taxes and the special Chicago Boutique Gasoline). The table below has each gasoline purchase numbered and each road segment labeled with a letter (for the trip back the locations reverse for these letters). In one case, I skipped a gasoline buying stop and those purchases show two letters. The trip back bought all 91 octane gasoline until the fill-up at #9.

A = Colorado Springs to North Platte, Nebraska (87 octane going out)
B = North Platte to Omaha, Nebraska (no stop going out)
C = Omaha to Des Moines, Iowa (89 octane going out)
D = Des Moines to Davenport, Iowa (89 octane going out)
E = Davenport to La Salle, Illinois (93 octane going out)
F = La Salle to West Suburban Chicago and City driving there
G = West Suburban Chicago to Davenport, Iowa.
H = City driving in Colorado Springs after return.

I’ve also calculated the average octane (AvOctn) for the gasoline in my 20-gallon tank after each fill-up and posted that in the next line down to describe the octane of the gasoline being burned on that segment.

Segment _ AvOctn _ Miles _ Gallons _ AvMPH _ Computer _ Calculated
1 - A) ____ 85.0 ___ 352 _ 13.386 ___ 55 _____ 23.7 ____ 26.3
2 - B&C) __ 86.3 ___ 398 _ 14.985 ___ 72 _____ 24.3 ____ 26.5
3 - D) ____ 88.3 ___ 167 __ 7.214 ___ 72 _____ 25.4 ____ 23.2
4 - E) ____ 88.6 ____ 91 __ 3.984 ___ 69 _____ 25.7 ____ 22.7
_ - F) ____ 89.5 ___ 295 ___________ 32 _____ 24.0 ____ NA
5 - G) ____ 89.5 ___ 153 _ 18.032 ___ 63 _____ 24.6 ____ 24.8
6 - D) ____ 90.8 ___ 167 __ 7.942 ___ 70 _____ 23.2 ____ 21.0
7 - C) ____ 90.9 ___ 145 __ 5.860 ___ 71 _____ 23.6 ____ 24.8
8 - B) ____ 90.9 ___ 251 _ 11.103 ___ 75 _____ 23.1 ____ 22.6
_ - A) ____ 91.0 ___ 322 ___________ 64 _____ 24.5 ____ NA
9 - H) ____ 91.0 ____ 41 _ 13.783 ___ 16 _____ 21.7_____ 26.3

Average 1 thru 4 ___ 980 _ 39.569 ___67.7 ____ 25.10____ 24.76
Average 5 thru 9 ___ 926 _ 38.688 ___ 67.0 ___ 23.62 ____ 23.94
Difference in gasoline mileage ________________ 1.5 ______ 0.8

Factors beyond my control that may have an influence on gasoline mileage. The difference in elevation between Colorado Springs and Davenport is 5900 feet – favors better mileage on trip East. This trip is almost an East to West trip, but not quite. Wind directions are given with the estimate of wind mph being very rough. The trip East had about a 10 mph wind from the SW for 1 and 2 – favors better mileage on trip East. The trip West had a 20 mph wind straight from the North for 5 thru 8 – favors worse mileage on trip West. After being in the Chicago area for a week, I added about 3 psi to each tire to get the cold tire pressure up to 35 psi – favors better mileage on trip West. Gasoline with 10% ethanol has less energy and should generally get poorer mileage. While I know that the 89 octane mid-grade had ethanol, it was not clear if the other gasoline I bought had ethanol, or not. So I can’t determine if this was a factor, or not. The vehicle speed and cargo weight were about the same for each direction. Also, during segment A on the trip West I noticed that the “Service Engine Soon” light was on. I’m not certain when this light came on. The engine was fixed (covered by emission warranty) after #9 above. The dealer’s invoice stated: “Found ECM needing to be reprogrammed per NTSB03-067A Technical Bulletin.” The Service Manager said that this should not have had an influence on gasoline mileage, but I’m not certain.

Why was the mileage burning premium gasoline worse than the mileage for the cheapest grade? I think the wind had the biggest influence. The cross wind on the trip West has a tendency to make the engine work harder than had there been no wind. And the slight tail wind for part of the trip East helped the mileage. All of these mileages were disappointing versus the 29 mpg readings I’ve had with this car driving this direction in the past. Perhaps the Service Engine may have had an impact on all of these mileages.

Because I’m not happy with these results, I will do another test this coming July for my trip to SC. I will take the same roads and compare the mileage I get with 91 octane gasoline in 2006 with the comparable mileage I got with the cheapest gasoline in 2005. Hope those results are more definitive.

Late Comment: See posts 20 and 21 in this thread for more information on this topic (and premium for city driving). I no longe plan to do the July (summertime) test discussed in the paragraph above, based on the helpful comments from SteVTEC in post 20.

Bobo

I was under the impression that it takes a few tanks of higher octane gasoline for the ECU to adjust to the change. You can conduct these brief tests until you are blue in the face and you will not get any definitive results.

You know your mpg when home. You know your mpg from this road trip. Next time IN BOTH DIRECTIONS run 93 octane gasoline, or 91 if you can't get 93 or are too cheap to buy it. Then report back.

99grnmaxgxe

iTrader: (11)

I was always under the impression that 87 always did better on highways and 91 did better in city driving.

Bobo

I was always under the impression that you need lower octane in the winter time than in the summer time.

SilverMax_04

Somehow I knew you would be the first to reply. If the ECU does not fully adapt until after 3 tanks of a different grade of gasoline, I will be in trouble every time I head East to a lower elevation. I've never had a problem doing that because the ECU adapts as I drive to lower elevations. Therefore I suspect the ECU adapts (as I drive) to different octane levels in the gasoline. Good try -- but wrong.

As for the other questions. In the old days (before computer controlled engines) you would get your best gasoline mileage when you burned gasoline that was ready to knock -- and would lightly knock on a few occasions. This octane would give you your best mileage (Engine Lab 101). There is a bolded statement in my 2004 Owners Manual (page 9-4) that makes me believe that this same rule may apply to the computer-controlled VQ Engine: "Now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is not cause for concern, because you get the greatest fuel benefit when there is light spark knock for a short time under heavy engine load." I take the "greatest fuel benefit" to mean the "best gasoline mileage." But threads on this site led me to believe (and other car's advertisements even say it) that you will get not only better performance but better gasoline mileage by burning premium. I was hoping to prove or disprove this belief about premium giving better mileage in a VQ-powered Max.

Here in Colorado at 6 K feet elevation, 85 octane acts like 91 octane at sea level, so I don't need 91 octane premium for city driving here. I only need to consider premium when I go to lower elevations. My next test of premium at lower elevations will be this coming July when I drive to the beach in SC. I will start off by filling my tank here with 91 octane so I start with an average of at least 89.5 octane in my tank (about 1/4 tank remaining when I fill up with premium). I will drive all the way to SC buying only premium, and see how this compares to my trip last year when I mainly bought the cheapest grade on that trip.

SilverMax_04

Heat is one of the major causes of engine knock. So you are correct, if you are burning premium to eliminate unwanted engine knock.

alanalan

hey silvermax 04......sometime i hear mah car pinging or knocking .i aint sure if it the engine or just the metal .........however it happen when i turn off mah engine i still hear these noises......is it the engine or metal?......and wut cna i do to stop this.....i was told to put tefron or something and put premium gas ..... (which i alway have)....wut is ur solution?i cant tell if it it the metal making the noise since it cold or the engine..oh this might be wierd question outta no where....hmmm i know this sound embrassing but i left mah ebrake on while driving on the highway i realize when i was in the highway.....but i went for a good 10 mins around 80mph.....wut will happen since i did that? i know it sound dumb...and sorry for the noob question.....and if i have to do it anything how much will it cost? in addition y do lower octane better like higher octane in a higher elevation and how did u avg the octane ......by mixing two differnet octane with the amount of gallons inside?

SilverMax_04

This sounds like you have engine deposits that are causing knocking (pinging). Options in order of increasing cost and time:

1) Put a bottle of good quality fuel injector (or even fuel system) cleaner in your gas tank. I like Chevron's Techron. Put it in your tank just before you fill it with gasoline and run the tank down to at least 1/4 before filling it again. Be certain to buy a big enough bottle to cover the capacity of your gas tank when it is full. With your problems, you may need to do this twice.

2) Replace your spark plugs. Most pinging occurs from the spark plug before the spark hits it. Thus, you are likely to have deposits on your plugs. Replace them.

3) If these other two don't work, you can take the engine apart to clean the valves and cylinder heads of deposits. This step will probably not be needed.

There is also the outside possibility that the spark advance system in your VQ engine is not working properly. You may want to have a dealer check it's function before doing 3) above.

The parking brake in my Max is a small drum around the hub of the two rear wheels. When you have the parking brake on, the brake shoes inside of this drum will expand to press against the inside of the drum. By doing what you did, you obviously heated up this drum and it was probably glowing red. Other than wear down the brake shoes, the only other damage you may have done was to "cook" the bearing lubricant for your two rear wheels. Given the relatively long time you were doing this (10 minutes), you probably should consider replacing the lube for those rear wheel bearings. At the same time, you (or the service tech) can look at the condition of the parking brake shoes.

I don't understand your first question here -- explain more. As for calculating the average octane of the gasoline for each segment of my trip, that was relatively easy. At the start I knew that I'd been using 85 octane long enough that this was what was in the tank for the first leg. When I filled up the first time, I put 13.386 gallons of 87 octane on top of 6.614 gallons of 85 octane gasoline (20 - 13.386). So I multiplied 87 x 13.386 plus 85 x 6.614, added the two together and devided by 20 (the total in a full tank), which gave me 86.339 average octane (I rounded to 86.3). Each time I filled up, I knew how much I bought and thus how much was remaining in the tank of the old average octane from the last calculation.

DrKlop

iTrader: (3)

Actually, I doubt that leaving the e-brake on will do anything serious. In most cases all you need to do is to readjust your e-brake setting. The same thing happened to me and some other people I know.

SilverMax_04

The small drums used in e-barkes for 4-wheel disc braked cars are not very large and thus do not dissapate heat very well. Depending on how hard these brakes were set, the little drums may well have been glowing red after 10 minutes at 80 mph. Some of that heat had to get to the rear wheel bearings and likely "cooked" the lube for those bearings.

If you don't replace the lube on the bearings, watch them carefully for premature failure. If what I feared happened, did happen, these bearings will fail in about 10 K miles or so. The first indication will likely be a rumble from the rear wheels as the steel in the roller bearings starts to break down. At that point, the bearings will need to be replaced.

Bobo

I'm glad I didn't disappoint you by replying. I said I would audit your results.

As for the differential between heading east and west, apart from wind I am sure the change in elevation heading west to Colorado has an effect on the mpg!

SilverMax_04

Both you and my Dealer's Service Manager believe that an elevation difference of about 5,000 feet between start and finish will cause a noticable difference in gasoline mileage. Those who have driven I 80, I 76, Colo 71 and US 24 between my start and finish know that the highway goes up and down driving in either direction. The only difference between driving east and driving west is that, after all of the up and down in each direction, there is a final elevation difference of about 5 K feet. While this could impact my mileage by a few tenths of a mile per gallon, I don't believe this has a very big effect on final gasoline mileage.

If I had to drive up a steep hill that had this much elevation difference in only 50 miles or so, I believe the mileage difference between going up and going down would be substantial. But for that elevation difference to occur over 926 miles or so should not make that much difference.

(You should also give me credit for mentioning all of the factors that could have influenced mileage, including elevation.)

alanalan

oh my question was why does elevation plays a factor going up a hill and down for overall gas mileage? its basically rolling down a hill (using less gas) and using more hp going up a hill i am guessing right?

SilverMax_04

Listen to you engine when you are going up a hill. It is working harder than on a flat road, and much harder than when you are riding down hill. When going up hill, you have to step down more on the gas to keep the speed from dropping -- using more gasoline.

One point I did not mention in my earlier posts. Most of the highway time for this mileage test was done with the cruise control working and set to between 7 and 9 mph higher than the posted speed limit. I've found at this speed that most highway patrol units will not stop you for speeding -- they appear to be looking for those doing 10 to 15 mph over the limit, or higher.

SilverMax_04

The point has been made on another thread in Fluids & Lubes that winter gasoline mileage this winter is worse than it was in past winters. Because I made the test trip to the Chicago area both last winter and this winter (with good records of both trips), I've decided to compare my highway mileage for both winter trips. I decided to post the results here because this is actually a continuation of this original post. Also, given that the East bound results are close to the same in both years, I'm starting to believe it may show (at least for winter driving) that premium gasoline actually gives worse mileage than the cheapest gasoline.

The difference in the two trips back and forth to Chicago: Since last year, the Interstate Highway speed limit in Iowa has been raised 5 mph in all but urban areas to 70 mph. So the driving in Iowa this winter was 5 mph faster than the previous winter. Also, in the previous winter I used the cheapest gasoline in both directions, while I used premium for the trip back this winter. For both trips I relied on the cruise control to give the engine gasoline, setting it at from 7 to 9 mph above the posted speed limit.

For both trips I have attempted to eliminate most city driving so that we can focus on the highway mileage for the 04 Maxima. The trip this winter, I filled up the day before the trip and then drove home. The previous winter I filled up on my way out of town the day we left and on our way into town on the way back. This past winter I made an extra highway gas stop in central Illinois that I did not make the previous winter and drove around town for 2 days before filling up on the trip back.

Here are the totals for both winter trips:

Year & ___ Total __ Total ______ MPG _______ MPG
Direction_ Miles __ Gallons __ Computer __ Calculated

Nov-Dec 2004
East ____ 902 ___ 34.634 ____ 25.5 ______ 26.0
West ___ 894 ___ 34.676 ____ 26.6 ______ 25.8
Total __ 1,796 ___ 69.310 ____ 26.1 ______ 25.9
Ending Mileage on car = 26,600

Dec-Jan 2006
East ____ 980 ___ 39.569 ____ 25.1______ 24.8
West ___ 926 ____ 38.688 ___ 23.6 ______ 23.9
Total __ 1,906 ___ 78.237 ____ 24.4 ______ 24.4
Ending Mileage on car = 42,600

Here is what has happend to my Max between these two trips: New Air Filter and Trannie fluid at 29,900 miles; new tires at 32,300 miles. The cleaner air filter this winter should give better mileage, the newer tires with more tread on them may have hurt mileage this winter.

While the results of the two different years are similar, the mileage last winter is definitely better than for this winter. I'm at a loss to explain why.

99grnmaxgxe

iTrader: (11)

I think Alanalan was talking about the metal cooling after he shuts off the engine. This is not knocking or pinging, it's just the metal cooling down. It always happens to me when I drive for a while and turn off the engine, it makes this noise.

alanalan

I can't tell, because its in the front so i am not sure, anyway.......question is.....why our cars and not other cars? just got me wondering.....does the type of brand of gas you used affect mileages?.......for example mobil vs bp? or a no brand gas station vs mobil? and the type of oil you used?

99grnmaxgxe

iTrader: (11)

Gas brands might affect mileage, take Chevron for an example, they have techron in their gas, so you might get better mileage from them compared to a no name brand. IMO Circle K gas burns up quickly compared to Diamond Shamrock (I mainly use these two brands). I think I get better mileage when I use Diamond Shamrock, b/c everytime I use Circle K gas, my mileage goes down.

kingrukus

iTrader: (41)

for me, premium gives better mileage vs 89 octane (the lowest octane I have used in my car before the motor was changed) for both highway and city driving.

SteVTEC

a new max (6g) shot back up my list after the Milwaukee auto show today, so I surfed into the 6g forum and saw this linked from the mpg thread and had some comments.

Actually that's exactly what I'd expect, better mileage on regular fuel. But even with that, your test still has far too many variables and is causing a lot of unknown in your data as you mentioned. The variable wind, drastic change in elevation, changing of fuel grades without knowning or understanding preciesly how/when the engine adjusts, etc.

In academic theory, regular fuel should give better mileage on straight highway cruising than premium. During highway cruising the engine is loaded extremely lightly which results in relatively low peak cylinder pressures even with the ignition timing at full advance. With premium fuel in the tank and those conditions, there is still plenty of knock margin left. So that allows you to switch to regular grade gas with lower knock resistance that will place the combustion process closer to the knock limit which is optimal. The other problem with premium gas in extremely light load conditions is that cylinder pressure and temperatures are never achieved to burn premium fuel all that efficiently, and that can result in more tailpipe emissions and also lower fuel economy. And I think this is why Cali has a max of 91 octane available. When you're sitting idling in traffic, 93 octane just isn't going to burn that well with a load of 0% even on a 11.0:1+ compression engine. Lastly, and since you worked in the oil industry you might be able to confirm this, I've read that regular grade gas actually has a slightly higher energy content than premium due to less octane per unit of fuel mixture (when octane tries to prevent or slow down combustion).

Of course, when you step on it now there's significantly higher peak cylinder temperature and pressure and now you actually need the higher octane fuel. But if regular is in the tank the engine will be past the knock limit expecting premium fuel and retard the timing back resulting in more fuel being burned (with regular) to produce the same amount of power. So if you do a lot of city driving with acceleration from a dead stop up to 30-45 mph a lot now you might actually get better mileage on premium fuel. But this thread is about highway mileage, not city. Lots of moderate acceleration favors premium for both power and efficiency. Lots of dead stop idling favors regular. Depending on your specific city driving mix either might give better mileage, so YMMV there. Highway is much steadier and easier to figure out though.

I work in the Chicagoland area and have family out in Minnesota and I'm out there most weekends. It's 333 miles one direction or 666 round-trip so I've had plenty of opportunity to test highway mileage under various conditions. There isn't a whole lot of elevation change, and I leave on Friday and usually come back on Sunday so the weather conditions are usually about the same too. Overall, whenever I use regular grade fuel I get about 1 mpg better overall, and it's for the reasons described above. There is no timing retard during cruise, the regular fuel places the combustion process closer to the knock limit which is optimal, regular grade gas burns more optimally in very light load (low pressure & temp) conditions, and the higher energy content of regular (if true).


Actually he could be right, because modern ECUs are very sophisticated and can be programmed in about a zillion different ways.

On my 4th gen, we're talking about mid-90's computing technology so it's relatively primitive. On the 4th gens, there is one ignition timing map which is set for premium fuel. You can run regular in it just fine and nothing will ever happen to the engine because the system will just pull back timing, but the result is a lot of lagging and surging in city driving. It keeps trying to run the full timing advance it would be able to achieve on premium, but since regular is in the tank the knock limit is constantly reached/exceeded and timing is retarded (lag) and then it eventually tries to readvance (surge). It's annoying enough that I just keep premium in the tank if I'm doing a lot of city driving.

Modern ECUs are a lot more sophisticated and can have a lot of different maps setup, including ignition timing maps, e-throttle maps, torque distribution maps for AWD cars, etc. Specific to fuel, two maps can be setup, one for regular and the other for premium. I'm not sure about Maximas, but on the 03+ Accord V6s if knocking is detected the ECU assumes regular fuel is being used and will drop the timing back for 300 miles. After 300 miles, the ECU will try to re-advance to the premium fuel timing map and will stay there until it detects knocking again at which point it will assume regular fuel (or extreme conditions) and drop back to the regular map again. And this is in the factory service manual. So for the people on the Accord site that usually run regular, this is why we recommend that they run at least two tanks of premium before doing any dynoing or going to the track. This is done to keep the engine operating smoothly regardless of fuel type or conditions. If the ECU is constantly trying to adjust the timing maps and going from one to the other the result is going to be complaints about lagging and surging which is not good. To confirm how precisely the VQ35 operates you would need to either page through the service manual to see what you can dig up, or do a ton of data logging with an OBD-II scanner. I put together a timing map for the 4th gen Maxima while having fun with my flakey knock sensor, which is linked in the 4g FAQ threads.

The ECU will adjust continuously for proper A/F mixture, and that's certainly needed when coming down from Colorado on a highway trip and then going straight back up. As for timing map adjustments based on fuel octane level, that may in fact take a certain number of miles, possibly in the hundreds.

SilverMax_04

Great post, Steve. It is great to have you back on this site. I have not seen a post from you in more than a year.

I believe you have convinced me that I should get as good or better highway mileage burning regular than with premium. So I will not even waste the time or money to do a summer test burning premium on my trip east to SC this coming July. You are certainly correct that (unless you floor it to pass) the cylinders are not seeing maximum load and pressure conditions with highway cruising -- ideal for burning regular. I will comment on some of your other points.

I certainly agree that there are a lot of variables in my test. But that is the problem with trying to measure fuel economy -- there are almost always a lot of variables. The only good way to do this test is to have two of exactly the same vehicle drive the same route at almost the same time and speed and check the difference in mileage -- not practical. I disagree that driving between Davenport, Iowa and Colorado Springs is a drastic elevation change. There is a change in elevation of 5,400 feet in a distance of 880 miles for an average of about 6 feet per mile. There are no steep hills on this trip and the highway goes up and down driving in both directions. A drastic elevation change would be driving west on I 70 from the west side of Denver to the Eisenhower tunnel, a distance of about 45 miles and the elevation increases from about 5,300 feet in west Denver to 11,013 feet at the east portal (5,713'). This gives an average elevation change of 127 feet per mile. The road is almost all up hill (but close to the top there is some up and down). Now that's what I call a drastic change in elevation. This stretch of I 70 is used by a number of manufacturers for testing vehicles for high altitude and for going up and down hills.

Correct -- that is Engine Lab 101 in Mechanical Engineering: "You get your best efficiency (and mileage) when the fuel you are burning is right at the edge of giving light engine knock." Others on this site have led me to believe that while this was true for old technology cars, that the sophistocated fuel and combustion controls on the Max would be able to provide better fuel economy burning premium to take advantage of the higher compression ratio and reduced need for a retarded spark. I think Nissan effectively agrees with you and Engine Lab 101 because they say in the 04 Owners' Manual: "Now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is not a cause for concern, because you get the greatest fuel benefit when there is light spark knock for a short time under heavy engine load. (page 9-4)"

It is tough to generalize about gasoline energy content because each batch of gasoline (even from the same refinery) is a unique blend of many different blending components. I do know that before the introduction of MTBE and/or ethanol into the blending equation, many oil companies blended a component known as heavy ultraformate into their premium gasoline for added octane. The heavy in the name indicated it was molecularly heavier than most other gasoline components -- thus it contained more energy per gallon. I guess, not having direct knowledge, I would have to give the likely higher energy nod to premium, but not by much.

Again, thanks for your post. It is good to "talk" with someone who knows the technology and how to express it in words. That is so much better than: "I will only give my baby the best -- premium gasoline." For those in this second camp, I want to be the first to agree that you had better be burning premium (now it looks like for at least the last 300 miles) before you race your Max or dyno your Max.

russianguy

iTrader: (1)

thats some serious research