ghost722

Ok, I know general consensus is that you shouldn't pay too much attention to the computer's mpg calculations, but since I've purchased the car in march, the dte has always said at least 330 after filling up and it has been pretty accurate. However, I recently had my first oil change(mobil1 oil and filter) and my first time filling up since then the computer said 240 and the mpg has gone from 19 to 11.(I know I will need to calculate by hand, but this seems like a huge drop)

The car runs fine just like it always has and my driving habits haven't changed. I checked the oil level and my tire pressures and those are fine too. Anyway, has anyone ever had this happen with their max or does anyone have any other things I should probably get checked out? Thanks (I've attached a pic for reference - supposedly 189 miles to go with almost a full tank)

Ghozt

If you've been driving it hard the computer will re-estimate with your most recent mileage stats. For example if I reset everything and go hit a highway trip averaging 70mph my tank will say 400+ miles to empty. However day to day it isn't that high for me.

dr_2010SV

Also did it to me once after a battery change.

jeffislouie

I've found that the computer's MPG hasn't been off by more than a few ticks. Distance to empty, however, is a load of bull. It is almost always dead wrong. My last fill up, the DTE showed 69 miles. I only managed to squeeze in just under 16 gallons and had an mpg of 21.2. This means that I had approximately 85 miles before I was empty (if not more - highway driving returns higher mpg than city only). I've been driving in stop and go traffic jams and watched that stupid DTE number go up, so you just have to know its bunk. One time, the DTE was 0 miles, but when I filled up, I put 15.5 gallons in, meaning I had a gallon and a half in the tank.

My previous car was an 05 Accord that held 17 gallons and I never managed to put more than 14.5 gallons in that thing, and that was one time when the needle was below E and I thought the car might conk out. It never, ever, ever ran dry.

I think the DTE is there as a guideline. It's a feature I've never cared for or wanted. I think it's totally useless. When my tank gets close to empty, I fill up. I don't live anywhere near a part of the country where I would have to plan a special trip to fill up. It's sort of a silly guesstimate the computer makes to prevent idiots from running out of gas.

My2010FoUrDsC

I also reset everything after filling up. Upon filling up last night, my average for that tank of gas was 20.4 mpg. It took 13.4xx gallons to fill up from 1/4 tank of gas. I drove mostly city and a few times on the freeway. I notice when I do city driving after filling up,my dte will say 385 but if I do highway the dte after filling up will say 420.

lightonthehill

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One time, the DTE was 0 miles, but when I filled up, I put 15.5 gallons in, meaning I had a gallon and a half in the tank.
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Since the 7th gen Maxima has a 20 gallon tank, I assume you meant you still had a gallon and a half before getting down to the last two gallons in the tank, because we should not be using those last two gallons, as the fuel pump is inside the tank, and requires being submerged in gasoline in order to avoid overheating. Also, if we run the tank very low, we suck in anything that may be floating on top of the fuel.

Nissan helps us avoid using these last two gallons by giving us low fuel warnings in several ways when the fuel level drops down to the the last four or five gallons in the tank.

We also must remember that running out of fuel in this car is a lot different than running out of fuel in the old days. Running out of fuel in this car can damage the catalyc converter to the point it will not pass inspection. As those who have had their catalyc converter stolen (currently a rampant crime in the metro Atlanta area), the replacement cost is now usually over $1000, over $2000 for some larger vehicles.

As to distance-to-empty, as others here indicated, this figure has to vary wildly because the car has no way of knowing what type of driving or driving conditions we will face during the remaining driving we will do on this tankful. So it lends great weight to the most recent few miles, which may have been in city driving conditions (15 MPG?) or drifting down a long downgrade on open road (50 MPG?).

There is also the factor of the fuel sensor not being in the exact center of the fuel tank, so readings are affected by whether the car is climbing, descending, or leaning left or right.

Because the car's computer has no way predict upcoming driving style or conditions, there is no way it can accurately predict DTE, so we need to understand how this number is created, and be very careful in trying to use this DTE calculation.

jeffislouie

You were close. I had a brain fart and for some reason used the size of my last car's tank instead.

Otherwise, you make great points. Running the tank down to empty is never a good idea. Most cars build in a little bit of a reserve by positioning the float in such a way that the car will show empty even though there is at least 2 gallons left. Honda's fuel gauges are notoriously off in this regard. It wasn't unusual to have the gauge read at the E line when there is still 2-3 gallons (or more) left in the tank.

lightonthehill

Right. It is sometimes interesting to note what the fuel guages and warnings are saying right before we fill up, then note exactly how much gas it takes to fill the tank. We usually see a discrepancy, sometimes larger than expected.

I have posted here before why I usually don't let the tank drop below 1/4 full. Way back over fifty years ago, I was in a position where I frequently drove coast-to-coast. I had become very proficient at knowing exactly how much gas was left in my tank, so I often 'stretched' things fairly close.

One afternoon I arrived in a little town in west Texas rather low on fuel, and pulled into a station. Turns out a strong storm had passed through town shortly before I arrived, all power was out, and no gas stations could pump gas. It was 60 miles to the next town (that is 'close' in west Texas). But back then, my big powerful car got 8 MPG in town and 11 on the highway (times really have changed). I had to check into a flea-bag motel which had no AC, lights or warm water due to the power outage. Couldn't leave until power came back on next day. Lesson learned.

jeffislouie

That's an awesome story that definitely proves your point. Don't stretch too much because you never know.

In general, I agree that filling up before your car shows E is a great idea and it's one I practice. There is no sense in pushing it anymore, especially considering the potential for damage to the vehicle.

Ah, the old days. One of my favorite things to do used to be rifling through my Dad's really old car magazines and looking up test results. For example, a 1970 Mustang Boss 302 managed a 0-60 time of 8 seconds. And that was REALLY fast for the time. It also would manage a whopping 11 mpg.

Always good for a laugh... Cars are faster, more efficient, and all around better handling than ever. They are also more complicated, more expensive to fix, and are chock full of idiot lights...

zman764

I've read this all over the Internet and have never understood the logic behind it. Gasoline is an extremely volatile substance, it vaporizes very quickly with no catalyst at all. The thought that engineers are designing fuel pumps to be cooled by being submerged in gasoline is almost laughable.

A more plausible explanation for why people have had fuel pump failures when running tanks out would be something along the lines of; a high pressure pump pumping a dense liquid such as gasoline suddenly sucks in low density air has a massive decrease in the resistance placed on the motor spinning the impellar resulting in significantly higher rpm's than it was designed to sustain. This makes more sense for failures in fuel pumps in empty gasoline tanks.

My expertise is in medicine I am in no way an engineer or mechanic, just surmising possibilities.

Either way I agree with light's overall point. Running a vehicle completely out of fuel is foolish, you can easily be stranded and you're sucking up sediment and other garbage from fuel into your engine / fuel system(keep in mind gasoline isn't a clean substance).

As for the OP's topic, my personal experience in the 40,000 miles I've owned my 2010 is that my date and mpg calculations have been extremely accurate. Once I filled my car with 19.8 gallons (attempting to find my real range). Both trip computers counted down to about 10 miles of range before displaying "---"

Did the date get better after a restart of the car? Did you fill up with the vehicle on?