Anyone know what speed to drive at to best the best gas mileage?
I usually drive 80 mph. I get great mileage at around 80. Anyone else have input?

Here's an online Gas Mileage Quiz. Good Stuff if you don't know much about gas mileage.

Gas Mileage Quiz

 

If you auto doing 80 will cruise you like 3K rpm. stick is like 3200k or so.. So I guess crusing @ 65 will yield a lot better mileage

 

Wouldn't it just be 55? I mean the optimal mph that EPA estimates are judged upon and which manufacturers strive for in order to maintain their 'fleet' mpg.

 

i dont think it has anything to do with the speed but the rpm's your engine is in

i know if i go like 65 on the highway i cruise at 2k and at 80 at 2600rpms the milage is much worse at 80 then at 65 and this is all in 6th gear

but i think you have a 2k1, the numbers vary but the theory is the same..keep your rpms done and you will get better milage

 

Speed DOES make a difference, but it's because there is more wind resistance.

I agree though, the RPMs are the biggest factor. I ALWAYS get 23.XXmpg on the hwy, doing between 75 and 95.

 

i never cruise at 55 on the highway cause i would get creamed, but thats supposed to be the optimal speed for MPG. 72 with cruise control got me about 28 MPG this past weekend on a 150 mile trip. that about as good as it gets for max

 

I just did an unscientific study about this same issue to prove a point to some "know it all's". I used me wifes '97 3.8l V6 'stang as the test mule...;-) The car has 65k miles on it. It was done on 2 back to back days, all driving patterns were the same. Same temp, time, and destinations back and forth. Here is my stats and you make the judgment. (All this was done to prove that the EPA is full of ****)

DAY 1:

filled up with 14.3 gallons
average speed was 80mph, engine rpm was 2.1k
time to destination, 40 minutes
time from destination, 30 minutes
miles on that tank of gas, 310
average miles per gallon, 21.67

DAY2:
filled up with 14.2 gallons
average speed was 60mph, engine rpm was 1.25k
time to destination, 65 minutes
time from destination, 40 minutes
miles on that tank of gas, 266
average miles per gallon, 18.73

I am not good at math, but it looks to me that my gas milage was ~20% better at a higher speed and I spent 10-15% less time on the road. My numbers speak for themselves. You guys can come to your own conclusions.

 

When you're highway driving, try not to let the car downshift (or if you drive the manual, don't go under your highest gear). It's kinda common sense stuff, but it's good say aloud anyway...

 

 

thats strange - i always get better mileage at lower speeds on the highway. also, you were at 1250 RPM goin 60? thats pretty low considering at only 20 mph faster you had nearly 1000 RPM more...

 

i dont know what the hell is wrong with your mustang(nuff said its a ford) but you shouldn't be driving at 1250rpms at 60mph and also at 80 the rpms sound very low.

i could drive my max in 6th gear at 20mph and the rpms would be under 1k but that wouldnt be normal driving would it

 

I might be a little off on the rpm's, but I know for fact in the stang that 80mph is 2k-2.1k rpm's. 60 mph might be closer to 1.5k rpm. As for whoever pulled out the BS flag, go out and try it. It needs to be 95% freeway or better. My wife was with me on both tanks of gas, she is the one who did the mpg calculations. Also, look at it this way; I can do a lot of city driving and keep all of my acceleration at or below 2.5k rpm and get 18-19 mpg, hit the freeway and cruise at 85-90 mph with 3.2k rpm's and get 22-23 mpg. You have to look at it from the fact that you are covering much more distance for a marginal increase in gas consumption. Could we get a mathmatician in here to crunch some number for us? If you still don't want to believe, oh well. I know what my odometer reads and how much it cost's to fill up every other day. Question to you unbelievers, how many miles a week do you drive, how many times a week do you fill up? I fill up every other day and average 350 miles a tank give or take 25. My driving habbits vary from mostly city to mostly highway on a daily basis.

A lie has to believable, the truth does not because it is the truth.

 

1.25rpm at 60mph?!, eh, i think that's wayy to low. something is werid about this

 

I used to think the same, but it is true. At 2k rpm's where my max is at 60 mph, my wifes stang is doing 80. Whats messed up is the fact that my max still gets better mileage, There is only 2 times that her car has ever gotten more than 300 miles to a tank. If I ever got less than 300 it will go to the dealer the next day. (shows you the impressivness of "domestic" engineering) And BTW, I put $25 in each tank every fillup and top off till I see gas in the hole. Go drive a friends '97 autotragic mustang or go ask on a mustang board if you don't believe me. www.stangnet.com (only tragic in this case 'cause the car is so god awfully slow).

 

This sounds like BS, but some weird stuff sometimes turns out to be true. Wait a second...it is BS. First, the RPM stated for the 60 mph can't be right. Second, the mileage in the run is about 105 miles - what about the other 150+ miles to drain the tank?

Anyway, I guarantee my 2k3 gets better gas mileage at 55 vs. 80 - don't forget that wind resistance isn't a linear curve. Can't remember the exact figures, but I saw some metric in a bike mag that wind resistance at 30 mph is something like quadruple the wind resistance at 20 mph (which is why it's HARD to ride a bike at 30+ for any length of time - unless of course you're Lance).

 

I forgot to mention that this was done twice in one day. Once in the morning and once in the evening, back and forth.

I am not saying that the 'stang gets better mileage, just stating that up until a certain point, it is more efficient to travel at a higher speed. Second, will YOU be willing to publicly eat crow when i post a picture of the car doing 60 and 80 mph? When was the last time you drove a mustang? It is awfully convienent to make factual statements about a car you have never driven. I drove this car over a year on a daily basis, you don't think I might have noticed or glanced at the tach a few times? My first car's speedo worked sporadicly and it became necessary to know what speed i was going by the tach, i thnk i still remember how to judge speed/rpm to within a few mph/rpm. Just don't go around calling BS when you yourself should be raising your own flag.

 

Incidentally i just posted some mileage threads over at the X.

Here's the initial thread about my "new record"

And here's the thread on my informal study/observations.

 

Why exactly would I "eat crow"? My specific statement was that my 2k3 gets better gas mileage at 55 than at 80 - what does that have to do with a pic of your Mustang at 60?

And you say above that "i still remember how to judge speed/rpm to within a few mph/rpm". Didn't you say THIS in an earlier post? If you're going to argue with me, at least read the posts and don't contradict yourself...

And my point, BTW, was not that you were lying, it's that your conclusion seems wrong. There are a many other factors related to gas mileage, and as you noted yourself the test was unscientific.

 

You stated that the rpm's at 60mph "can't be right". If there is only one thing that I can prove, it is that. Everything else is just smoke and mirrors as far as anyone else is concerned. I watched the speedo, I watched the tach, I filled up both times and I drove both days. The numbers that I stated are what I observed, and the conclusion I stated is what I came to after seeing those numbers. You are very right, it was very un-scientific. There are many variables. It might have to do more with my own driving and mindset than any numbers I might have observed, who knows. I still stand by my conclusion that I get better gas mileage going 80 and not 60. In my first post I also stated for everyone to come to their own conclusions, not start throwing BS flags around because they either do not agree with or like my numbers or conclusions. I still don't understand how I am "contradicting myself". I only said that my rpm's at 60 might be off by about 200-250rpm. I see in no way how that is "contradictory".

 

can yo upost a digi picture of your stang's speedo and tach at 60 then???

i still dont think it is right, i have not seen any car rev that low at optimal cruising speed (55-60mph) I am assuming the stang is an automatic too..right?

 

Yes, tonight I will tape the 'stang going 60 and 80 from the drivers seat. I have to tape it and then rip it to video, and then capture the frames. Long process because our "community" digicam is MIA. It will be posted tonight. I would post the video but have nobody to host it.

 

Update, I have taken the video that I need. I will be posting the video frames up when I get home from work tonight.

 

guys, calm down... first of all, rob is right about the rpms... stangs are designed for maximum torque.. thats whay they are called "muscle car". their engines are not meant to go at high rpms. Most american cars are designed that way, especially Ford. They build their engines to produce the most torque and you achieve the most torque at low rpms. that is why stangs can take off from the line. but they have no high end power.
On the other hand, japanese cars are usually made to produce the most horsepower which can be achieved by having real high rpms. That is why american cars usually run at a lower rpm. that is because they are designed to. try renting a mustang or a camaro and see for yourself. You do not need to rev the engine to make that sucker take off. And also notice that thier redline is also lower then most japanese cars. (Note: Look at torque curves and horepower curves to see.)
Now stop acting childish and accusing each other of lying about what rpm a car has on certain speed. It will be different on every car depending on how their powertrain is designed.

 

oh, but about getting better gas milage at 80 compared to 60, That is not correct. All cars are designed to be most efficient at 55MPH. the higher you go, the worst your gas milage get. you did your calculations wrong. from your data, you drove for a total of 70 minutes(40 +30) round trip. but you have 310 on your total miles. that is not right. if you can drive 310 miles in 70 minutes, i will kiss your A$$ literally. you put the miles for a whole tank of gas but you didnt use the whole tank. I doubt your car is running on empty after a 70 minute drive, unless ya have a 4 gallon gas tank. hehe. also your milage doesnt equal up. 80MPH for 70 minutes is a 93 mile round trip. 105 min (65 + 40) at 60MPH is a 105 mile trip. If you take the same road, the miles driven on both trips should be the same. And last time i checked, 93 does not equal 105. Engineering major here so did everything quickly. sorry if you guys dont follow the numbers. But rob, your data is incorrect to start with.

 

Mclee, not to flame but i thought this would be an interesting discussion.

How would coefficient of drag and automatic suspension leveling come into play with the above statements?

I know several vehicle's have a system in place to lower the vehicle's ride height on highway speeds. I'm assuming that also helps minimize drag at higher speeds. If this were the case, wouldn't a higher (to an extent) speed make gas use equal to or less than what one would use if they were to go 10-20mph slower? (e.g. lower wind resistance and drag = less engine grunt)

 

automatic suspension basically tries to level the car to give it the least drag. less drag = more efficient car. but the vehicle lowering at high speed is not caused by the automatic suspension. the lowering of hte car is cause by pressure change above and below the car. automatic suspension just tries to keep the car at the same angle as it was when the car is not moving. lowering the car does helps minimize drag but not much. lowering actually has more affect on keeping hte car on the ground then it does on drag. the lower the car is, the faster the velocity of the air is at the bottom of the car. this high velocity air traveling beneath the car changes the air pressure underneath the car pulling the car down.

the automatice suspension basically levels the car to give the least drag. a car has the best aerodynamic when sitting still but once you drive the car, the front end of the car either rises and the back of hte car drops usually. this change in angle of the car(like dropping the front 3 inches and leaving the back stock) changes the drag coefficient of the car. the automatic suspension tries to balance the car back to its original position to give the best drag coefficient. its like sticking your hand out flat with the palm facing the floor while driving at high speeds. feel how small the drag is? but then change the angle of your hand by raising the front of your hand a little. you will feel more drag and it will even sometimes try to make your hand go up. what the automatic suspension does is like trying to maintain your hand facing the floor no matter how fast you are going.
your car might be at the same angle at 80 or at 60 but the drag coefficient is different due to the different speeds. faster you go, higher drag becomes. i dont know if you can follow what i am saying but this is the best i can describe.

 

I think i understand what you're trying to say, but from what i've read about these systems they not only effectively level out the vehicle but it actually lowers the entire vehicle (front and back) by as much as 2 inches.

If i'm not mistaken, you seem to be talking more about lift. My take is, if the ride height is lowered over-all there's less lift. Less lift would also mean less drag.

If that were the case, would you agree that higher speeds would then help make fuel efficiency at 55 = to that at 70 (example)?

 

if you lower the car, yes you get less lift and less drag... and you will improve fuel efficiency the efficiency you were getting at 55 before the car was lowered might be the same as going 70 with the vehicle lowered.. yes. but remember the efficiency going at 55 also gets better which still makes it better then 70

Efficiency comparison: (whats more efficient)
NON lowered car at 55MPH > NON lowered car at 70MPH
NON lowered car at 55MPH = Lowered car at 70MPH(maybe depends on efficiency gained)
lowered car at 55MPH > lowered car at 70MPH

For you to get the same fuel efficiency going at 55MPH, you have to lower the drag coefficient of the vehicle going at 70MPH by either dropping the car or making it more aerodynamic. but comparing both speeds with the same setup on a car, the slower car will be more efficient.

 

Let me clarify

The leveling system on the vehicles i mentioned above only activates at a certain speed (60mph IIRC). With that being the case, following your post above would that mean:

55mph -> good fuel efficiency
70mph -> leveling activates, lowers car -> similar efficiency as 55mph

Nice discussion

 

my old 94 t-bird with the same 3.8 was at 1950 rpms @60 mph.

 

yes.. that is assuming that lowering the car reduces the drag to where the drag force going at 70 is less than the drag force going at 55(which will never be true for a same body car). for the efficiency between the two speeds to be the same or close to the same, the drag on 70mph has to be lower then the drag of 55mph. this is because 55mph is burning less gas to begin with and is running at lower rpms. so to compensate for the higher rpms of going at 70mph, the drag has to be lower then the drag of going at 55mph. This actually proves that driving at 70mph will never be as efficient as driving 55mph because driving at 70mph, even with the drop on the car, will increase drag. drag caused by the 15mph speed increase is much greater then the drag force reduced by lowering the car. so the extra drag caused by the speed increase cancels the reduced drag by lowering the car and adds some more drag.

(added drag from 15mph increase)-( reduced drag from lowering the car) = added drag.

This is because the aerodynamics of the car stayed the same no matter at what speed so it doesnt change the drag coefficient much by dropping it. It does but not a significant amount. On the other hand, 15mph added to the speed increases the drag dramatically. the slope on a drag is exponential.

 

Ehh.. Physics 101.

Air drag is simply loss of kinetic energy due to hitting against so many tiny air particles. If we remember kinetic energy correctly, it's 1/2 * mass * velocity^2. So effective kinetic energy of the air against your car increases parabolically. Since your car has to replace the lost energy due to the air drag to keep the car running at the same speed, it has to get energy from the engine, which in turn gets energy from the fuel and gas mixture. Therefore the loss of energy due to air drag that your car faces increases parabolically - meaning a car traveling at 80 mph loses 4 times the energy due to drag as an identical car traveling at 40mph. Coeficient of drag is simply a coefficient - a constant that gets multiplied to the drag of the car if it was just a box. I think it's something like .35 for a typical maxima. Maybe like .46 for H2? I don't think the change in coefficient of drag due to shift in car's orientation is siginificant enough to offset the parabolic growth rate of the drag.

The other side of the equation is the efficiency of the engine at various RPMs. Generally, the lower the RPM above idle, the higher the efficiency is. Efficiecy of the engine is just output power / input potential energy, where input potential energy is the chemical energy produced from oxygen and fuel mixture. Generally, I think internal combustion engines are less 50% efficient but I could be wrong, I'm not an automotive engineer. Engine losses efficiency due to higher rpms due to friction. And the friction creates heat, sound, etc, which the input energy converts into instead of outputing power at the crankshaft.

And, of course, there are other efficiecies to take account such as heat generation in the transmission, axles, bearing, tires, etc. I think all those energy loss due to friction increases parabolically as well or at least faster than linear rate.

Therefore, the speed that'll give the best mileage is whatever the speed you get at the highest gear you have when your engine is going at like 1k rpm. So prolly for most cars, 40-50 mph. But then there's time efficiency, too, which is more important for me than a few bucks

 

WOW. All this is great information. Seems like we have a couple honor students.

About lowering. I bought the maxspeeds. Do you think I will get a good amount of gas mileage increase?

 

I know I saw this somewhere on the forum a year and a half ago, or so, someone had posted the fuel injection rates as programmed in the ECU. IIRC the lowest injection rate was somewhere in the 3000 rpm range. So the best milage, the lowest fuel usage would be in that range.

 

I agree, we shouldn't be arguing about rpm on a specific car, but i did get my pictures up. http://members.cardomain.com/robsae It is on page 5.

I know I left some data out in the equation and that is my bad. The trip specified accounted for ~75% of my driving. The other 25% was filled with around town FREEWAY driving maintaining the average speeds as the main trip for that day. The around town driving was not exact in either day, but the trip was. The trip was done 2 times a day. I am also not arguing about what speed the engine runs most efficiently, the point I am trying to make is that with the overall higher speed I am getting better mileage AND spending less time on the freeway, which in itslef saves gas. Wouldn't I burn less gas doing 20mph more for 20 minutes less time? Also, since everything increases parabolically you are saying that my gas consumption should increase ~4x's at 80 than if going 60? I thought that for the marginal increase in gas consumption you made up for in distance travelled plus time reduced?

 

Rob,

Of course, the distance covered as a function of speed must be accounted as well to get the overall picture, my bad. The distance covered simply grows at linear rate. So if you travel at 80 mph, then you're twice as efficient as if you were to travel at 40mph - at the cost of 4 times the drag (80^2 = 6400, 40^2 = 1600). But then, again, the drag isn't the only factor that takes away your kinetic enery - but it becomes the dominating factor as the speed increases since it grows at parabolic rate. Most sports cars are drag limited.

I'm not saying your gas milage will double at 40mph vs. if you were to travel at 80mph. That's because there are many other factors - that I'm not all sure how they exactly interact. I'm sure some of them are constant (mass of the car), some of them are linear (rolling friction), and maybe some of them grow at even higher rate. But obviously the slower-growing factors dominate for lower speeds ( < 100mph, I guess), so that you'd prolly see 10-30% increase (I pulled the number out of my *** ) in gas mileage between 40mph vs. 80 mph at the same gear ratio.

But I'm very sure that if you compared gas mileage between 100mph vs. 200mph, the gas mileage will be very close to or even better than twice for 100mph - I think some of the engine power loss might be faster than parabolic, maybe even exponential? for high RPMs. I guess only Germans can tell you that. Now if you compared speeds of higher magnitude, then you'd have to account for turbulance and stuff like that. And if you travel at even higher speeds, we'll be looking at quantum mechnics and the gas efficiency looks REALLY bad there

 

Rob i stand corrected. thanks for posting the pics i didnt know mustangs reved that low.

again thanks for the pics, btw very nice MAX you have there

 

It's all relative to piston speed(fps) and gearing.

SteVTEC has done a few posts on how not all RPMs are equal.

 

I will be looking for that post. I want to see how 1 rpm is not equal to another rpm. might anyone have the link for that perticular post?

 

So, what you are saying in laymans terms (my language) that it is possible that there is a better mpg/efficiency to driving at 80 than 60? Or not that fast, maybe like 70 as to 55? For extra credit could you tell me at what point the curve starts to move away from most efficient? (( in terms of mpg, time, and distance travelled)just kidding).

I am glad that this thread did not turn into a flame war, as I have learned much. (like details make all the difference ) Still, can someone tell me why i can drive my wifes 'stang at 80 all day long at never get better than 21-22 mpg but i can do the same in my 'max and get 25+mpg? I have the heavier, larger, higher revving engined car and it just does not make sense. Or should I just chalk it up to good old fashioned American innefficiency...er...I mean inginuity?