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I'm driving a 2008 Altima Coupe 2.5 Auto transmission.

I believe the math in my car is 100% wrong.

The manual (and online sites) states that the tank is 20 gallons, yet when I filled up a while back, I was under a quarter of a tank and only got 15 gallons in. I also filled up yesterday when the gas tank read close to 3/4th of a tank and it only filled up 3.625 gallons.

That's my first issue. Does anybody know what the actual tank size of my car is?

My second issue is with my reader. It came with the car and is very useful as it tells me mpg, miles to go before I'm empty, and what my average speed is.

Well, I think it's jacked up as well. When I filled up my 3.625 gallons, my car read 37.8mpg. If you do the math, that should've been 137 miles. My car read 173 miles.

I usually get between 36 and 37 mpg, would should logically, if the gas tank was 20 gallons, give me 720-740 miles every gas tank. However, when adding what I've driven (540 miles) and how many miles I have to go (90 miles) it gives me only 630 miles.

What's going on with my car? Is my reader completely wrong? Does my car have 20 or 18 gallons in it? Is Al Gore controlling the weather??

Any help is appreciated.

Fuel gauges are notoriously inaccurate. And non-linear, too. I think all the cars I've owned, when filled up to where the gas pump shut off, could go a considerable distance before the gauge would start dropping. And likewise would have several gallons when the gauge was showing empty.

In my Insight, I generally go 700-750 miles between fillups. After a fillup, I can go at least 100 miles before the gauge drops one tick. And when I go to the last tick, so the fuel light comes on, a fillup will put about 9.4 gallons in a 10.5 gallon tank.

Fuel gauges are notoriously inaccurate. And non-linear, too. ...

Believe it or not, the same is true of private airplanes. I remember watching some of the Sportys videos when my wife was training for her pilot certificates. I remember them saying that the regulations state that the only time the fuel gage *must* be accurate is when it reads "empty". So the instructor stated that you could theoretically have a gage that reads Full until the last cup of gas, and then sweep immediately to "E", and it would be legal. Thankfully, those of us on the ground don't have as much to worry about when and if we sputter to a stop!

I also remember reading that sulfur in gasoline contaminates the sender in the tank and leads to all sorts of interesting readings on the gage. Some brands of gas have more sulfur in them than others and some brands of vehicle are more sensitive to it than others. So this could just be the result of a Perfect Storm of hi-sulfur gas and I-hate-sulfur fuel sender.

Most likely, that 20 gallon tank is probably set to read "empty" at 16 or 17 gallons to prevent drivers from running it dry and frying the fuel pump.

... The manual (and online sites) states that the tank is 20 gallons, yet when I filled up a while back, I was under a quarter of a tank and only got 15 gallons in. ...

The gauge markings may not be very accurate. Have you run it down far enough to find the bottom of the tank?

My three most recent cars have had a considerable portion of their tank capacity placed below E. The Subaru's tank is claimed as 15.9 gallons, yet E is only 11.5-11.8 gallons down. The low fuel light is about 13 gallons down, and I'm comfortable running to 14. Your individual car, even if it was the same make/model/year as mine, will have different numbers. My very first car accidentally ran dry just _above_ E.

The one car I intentionally ran dry, a long-gone carbureted model, went 110-120 miles below E, and I was comfortable using half of that. I don't have the courage to run a fuel injected engine dry.

But I do demand a certain fuel range of my cars, based on a long-ago experience on a long stretch with few gas stations, nearly all of them closed on a holiday. The one open station demanded $5 (that would be 5 gallons in today's money) to unlock the pump -- and no cost splitting allowed with other customers. The next open station was 135 miles away. I drove slower and went for it. Their overpriced gas never looked so good.

Ever since, fuel range and fuel stops (with opportunistic stops for cheaper prices) have been important planning items on every trip.

Don't try to calibrate your gauge all at once. If you are confident in a claimed tank capacity, decide how much of it you are willing to use, leaving some margin for hills and bad conditions. Then gradually run the needle down a little bit further than a previous fillup, recording the gallons needed to fill vs. needle position, until you reach your limit. Test new lows only in situations where stations and time and conditions allow you to screw up without serious consequence.

I don't know how much fuel becomes unavailable when the tank is tilted on hills. Math-o-phobes, and people unable to walk a few miles if get it wrong, should not attempt this.

Does anyone have any fuel system cautions about running to the bottom of the tank? I have ignored the common warning about sucking in debris collected at the tank bottom, preferring instead to stir it up often enough to get it flushed it into the filter and removed with regular maintenance, before it builds up enough to cause problems. But another source claims that the fuel in the tank is needed to keep the pump properly cooled for longer life. Is this the reason my Honda fuel pump needed replacement at about 200k?

In 14 years of driving, over several brands of cars (all but one fuel injected), I've never had an issue running 30 miles on LFL light. But, your mileage may vary.

I discovered yesterday that my LFL comes on with 2 gallons left in the tank. And a couple of fillups ago, I filled up when the gauge was just above E and still had just over 3 gallons left in the tank. Knowing that now, I'm going to push it further this tank, see how long I can go between fillups (made it 19 days this last tank).