Funky fuel tank... How do I really know how much gas in in my tank?

Hey guys, I have a dilemma. I own a 2000 Mercury Sable (same thing as a Ford Taurus) and there’s something fishy about my fuel tank. The owner’s manual, car spec websites and dealer documents all state that it has an 18 gallon fuel tank. However even when I brim the tank after driving around on E, I can never get past 13 gallons into the tank. Yesterday I drove 25 miles after the fuel light came on and the gas pump quit pumping at 12.9 gallons and I only managed to get 13.1 in after that. This is supposed to be an 18 gallon tank! So what gives?

Is my fuel level sensor malfunctioning and I’m driving around with 5 more gallons than I think I have? Is there something inside my gas tank? Ever since watching the show Border Wars, I speculate that there is either a bundle of cash or maybe some weed in the tank (I bought the car used). Even though 5 gallons is not that much, having a 13 gallon tank in a 14 year old car where the MPG has dropped drastically over the years can be a pain. Anyone have any ideas or ways I can get to the bottom of this issue?


It is very possible that the gas tank is dented, and, as a result, has less than its specified capacity. You might want to get underneath the car in order to see if the tank is dented.

I suspect that you may have a smaller tank. Maybe the production run changed and a smaller tank was used on some. Maybe it was replaced before you got the car. Maybe the car came with different size tanks and the 18-gallon was an option that yours doesn’t have. It would have to be quite the dent in the tank to lose 4-5 gallons of capacity. Possibly a dealer (or maybe there’s even a website) that can decode your VIN and tell you what the car actually shipped with.

I guess the only other way to really tell would be to run it bone dry and refill it. But that’s not really a good idea for many reasons. I would probably just stop worrying about it and pretend you actually have a 15-gallon tank.

I think your tank level float is mis-calibrated. The sensor is a simple variable resistor with a long, thin rod and a float on the end. The rod can bend and throw off the tank level sensor. The only way to know for sure is to run the tank near empty, then lower it to see how much is still in it. Or, see how far you can run on an empty tank until you run out of gas.

Any of the above noted possibilities is plausible. I don’t know your level of mechanical savvy, but if you really wanted to get to the bottom of it (pun accidental) you could wait until the fuel light comes on, disconnect a fuel line, rig it to a gas can, and energize the fuel pump and pump the tank dry. Once you know its empty, see how much gas you have in the can, put it back in, then fill up.

On a completely different note, its not normal for fuel economy to drop drastically unless there are things that need attention. If a car is kept in good shape, it’s fuel economy should remain pretty stable.

I agree with cigroller. How much has your fuel economy dropped? Are you up-to-date on all maintenance? Is the check-engine light on?

I agree with all the possible problems with your gas tank and I will add one more. If it’s a used car then someone could have changed the gas tank for any number of reasons.


  1. Cigroller is right that fuel economy doesn’t decline over the life of the car unless there is something wrong that needs to be fixed. I drive a 16-year-old car with 251,000 miles on the odometer, and I just checked my fuel economy on the last trip I took. My fuel economy was better than the EPA estimate that was on the sticker when I bought the car 15 years ago.

  2. Don’t top off your tank. You wrote, “…the gas pump quit pumping at 12.9 gallons and I only managed to get 13.1 in after that.” Why did you add the additional 0.2 gallons after the pump stopped? If you top off your tank often, you might have damaged your evaporative emission system (the charcoal canister through which your gas tank vents). This might explain why the pump stops before you fill the tank completely.

  3. Do you have an illuminated “check engine” light? I’m betting you do. This might or might not be related to this fuel tank issue.

I really appreciate all the responses guys! Thank you so much!

@Cigroller, the car runs great. No check engine light, I’m all up to date with the necessary maintenance and I just had an oil filter change about 500 miles ago. I’m griping about fuel economy because I’ve been driving the car in an extremely hilly town that requires me going up a lot of hills. With the automatic trans, the car doesn’t get to up shift as soon as it usually would on flat terrain, and I find my self in the 2500 - 3000 rpm range, which is higher than than usual shift point. Fuel says city driving should return 17 mpg, but when I last calculated it I was getting about 12 mpg. I try to keep my foot out of it too, so I’m not burning gas that way.

@BustedKnuckles I have thought about draining the tank and seeing where she cuts out, but I don’t want to bring any of the sludge thru the fuel filters when the tank gets sucked dry.

@oblivion The car is model year 2000 and was manufactured in 2/00. I know the Sable / Taurus was refreshed for that year mechanically and cosmetically, and platform that it was revised upon had been in production since 1996. The fuel tank for the generation before mine was 16 gallons, still leaving 3 gallons unaccounted for.

@Whitey I know I cringed when I topped the tank off and I assure you I never pump more gas in after it stops pumping! I did it this time to see exactly how many gallons I could get into it before I posted this discussion.

SO I guess my options are to run it dry and see if I do have 5 extra gallons, check to see if the tank is replaced, or maybe take it to a shop to check it out. Unfortunately I am not that mechanically savvy. I’m open to any suggestions that you all have! Thanks a bunch!!!

You won’t hurt anything by running the tank dry - but I would do it into a container instead of waiting to just run out of gas on the road someplace. I can’t tell you for the Sable, but there’s usually a hot wire under the hood someplace. Pull a fuel line, run a jumper from the battery and let the pump pump the fuel out into a gas can. That’s if you really want to know.

I would still look at things that might be pulling down the fuel economy. Hills or no, what goes up must come down. It can’t be uphill all of the time and as much as you’re getting terrible gas mileage up you’ll be getting great mileage coming down. I’m not saying it will all even out to what you’d get on flat terrain, but 12mpg is pretty low for that car. Coolant temp sensor? Thermostat? Old, tired O2 sensors?

I think hills and cold weather easily account for the difference between 15 and 12 mpg.

As for what goes up the hill must come down, coming down the hill you are using your brakes and turning all that energy you used going up into heat going down.

@oldtimer_11, even 15 is pretty low for a Sable. And whether you have to use your brakes a lot when you come down depends on how you drive. But it also doesn’t change the fact that if you do a lot of downhill coasting or near-coasting you get wildly high gas mileage - whether you have to use your brakes or not. So if you just climbed 1/4 mile while hammering it, you might be getting 5mpg or something. But on the way back down - well, IDK, all of the instant economy computers maxed out at 99mpg. But I sometimes have some fun on trips looking at these things and its frequently showing 99mpg on a downhill - whether I apply the brakes or not. Gravity is gravity.

If I was the OP, I would continue to look into the fuel economy. And if he’s getting as good as it gets then so be it. But I wouldn’t just sit around making assumptions.

Can you rule out the teenage neighbor with a siphon? Be a shame if a locking gas cap would fix all your problems. And I suppose its unfair of me to pick on teenagers, but that sounds like something I might of done a few years or decades ago.

@cigroller: “Hills or no, what goes up must come down.”

Then how did my grandfather walk to school uphill both ways? :wink:

I don’t know about anyone else. But I would rather the gauge read empty and still had 5 gals. in the tank than reading 1/4 tank with only 1 gal in the tank.

18 gallons was an option.
16 was the norm so I’d be highly curious about the dented theory.

It seems like this puzzle has to be caused by one of these three things

  1. The tank doesn’t have the capacity the OP thinks it does.
  2. The tank isn’t completely empty when the OP thinks it is.
  3. The tank isn’t completely full when the OP thinks it is.

It seems like if you drive the car until it stalls b/c out of gas, that’s about the best you can do for number 2. For number 3, it is possible the venting system isn’t allowing you to fully fill the tank. Maybe the vent line which normally goes to the purge valve or canister could be disconnected temporarily to see if it is possible to fill the tank more. This could all be damagaing to the car or be dangerous by the way, as running the tank out of gas can pull in debris into the fuel filter and injectors, can damage the cat, and gas fume or gasoline itself could spill out if you disconnect any venting hoses. Maybe better to just live w/it.

Just a bit off-topic, but I’ve been hearing this crap about running the tank low and the pump pulling crud from the bottom. Hate to burst your bubble, but every fuel pump I’ve replaced in all EFI cars pull gas from the bottom of the tank. It is already pulling crud, full tank, half tank, or near empty. I have yet to see a tank with a load of crud at the bottom of it. Some debris, sure. But not a good load of silt. The fuel filters, however, are a much different story.

My 2 cts, rant over. Thanks.

Cigroller, You do realize that if you get 5 mpg going up the hill and 99+ all the way down, your average is less than 10 mpg. Also, the OP stated the EPA city was 15. I have never had a car that bettered it’s city rating around town. Highway ratings I have beaten on the road but not city.