In defense of topping off

Hi Ray.
I just saw your article about NOT topping off the gas tank in our paper.

Topping off the tank is something that I have been doing for years.
I can usually put into the tank another 2 to 3 gallons on each fill up.
To my knowledge I have never had a problem with any of my cars.
Right now a have a Prius that I have had for 10 years.

The reasons that I do this is because I record my gas mileage every time
I fill the tank and want to know what my miles per gallon are that I am getting
for each tank full. I also track my overall average for the life of the car.
If the average starts to drop, then I can have my mechanic look at the car.
That has never happened, YET.
Not filling the tank, my mileage would very for each tank full. Maybe the over all
average would work out to the same.
Another reason I want to do this so that I do not to fill the tank as often. Saving
me trips to the gas station to fill up. This is very helpful on long trips.

Just wondering if I am missing something.

Enjoy reading your articles in the paper.

Fred K in Pennsylvania

Ray does not answer questions on this members forum that we know of, however the reasons he gave in his column are valid and I believe would be endorsed by almost all members.


You don’t need to fill the tank to the brim to do that.


The problem with topping off the tank is you run the risk of sending raw gas to the EVAP through the vent tube. This will kill an EVAP system in short order. Some vehicles are designed differently then others. Personally I wouldn’t do it. Just because it worked fine on other vehicles doesn’t mean it’ll be the same for any future vehicles.

As for gas mileage…I don’t see how that’s going to be a problem. Topping off the tank isn’t going to effect gas mileage that much. A windy day will effect gas mileage more.

Then you must live in an area that has the same weather and temps from week to week and month to month. Rain, snow, cold, hot all can have an effect on MPG.

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On earlier models topping the fuel tank allowed fuel in liquid form to get to the cannisters. This would eventually flood it causing all kinds of problems eventually. Manufactures realised this problem and redesigned the gas tanks to eliminate it. I don’t know when this fix was universally made so I like to play it safe and do not top it. I just got a 2020 vehicle and the owners manual specifically stated to not let the fuel nozzle to click over 3 times.

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By this reasoning, since my grandfather smoked cigarettes and lived into his 70s, I should be able to do the same. It hasn’t backfired yet.

Sarcasm aside, you don’t have to fill your fuel tank to the very top to track your fuel economy. Whether you fill your fuel tank 90% full, 95% full, or 99.999% full, the calculation you’re doing doesn’t change. You’re dividing miles traveled by the amount of fuel used to fill the tank. Sure, your per-tank fuel economy might not be as precise, but your overall miles traveled per fuel used will be the same whether you stop at the first click of the fuel pump handle or the third. (Actually, if you use the same fuel pump every time, you might get more accurate results if you stop at the first click.)

One last thing: If topping off gives you an extra 2-3 gallons, your car has issues. On the rare occasions when I’ve topped off (because I was driving across a desert or riding a motorcycle that has a small fuel tank in a secluded area), I’ve never gotten that much into the tank after the first click. You’re either dealing with a malfunctioning fuel pump or you’ve soaked your evaporative emissions recovery system with fuel so many times that it isn’t working properly. (I’m betting it’s the latter.)

In any case, nobody should try to emulate what you’re doing.


The difference isn’t all that significant. If you go 500 miles and then fill 10 gallons, you got 50mpg. If you fill 11, you got 45.5mpg. As long as you fill the same way each time, you should get sufficient results for determining if there’s an efficiency problem because you don’t need to get excited about it until it drops by 10mpg or so, and that will be reflected no matter how you fill as long as you do it consistently.

And to get that unnecessary level of supposed precision you have to spill gas from overfilling, because unless you do that you never know when it’s completely full. Spilling gas can damage your paint and isn’t great for the environment either. You also aren’t taking into account environmental factors which effect fuel volume (hotter gas expands and will indicate more gallons on the pump than cold gas) and burn energy (hot gas comes with hot weather, which means hot air, which means less oxygen per unit of air and therefore less gas used per cycle, which is why mileage drops in the dead of winter). And those two factors will have a larger impact on your calculations than overfilling your tank will. Not to mention that your reaction time might vary, which means you’ll spill a different amount of gas each time and that will throw your calculations off as well.

Plus, as @Whitey said, I have difficulty believing that you get an extra 3 gallons on an 11 gallon tank by filling past the first shutoff unless there’s something wrong either with the pump or your car.


If the OP refills consistently, say stopping after the first click, he’ll calculate the same mpgs.

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The Prius has a bladder inside the fuel tank that is known to shrink as it ages making it difficult to fill the tank. Hard to calculate the fuel economy when the fill is inconsistent.

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To the OP, if it makes you feel better, after the 1st click do it once more. And do it the same each time you gas up. you’ll still get good numbers and it’s better for the car. And your pocketbook since you won’t cause a problem. I drive a Chevy Volt. I don’t have to gas up very often. But when I do I don’t add any more after the 1st click.

Why take a chance on the unknown, so you can get another 20 miles before fillup. Forgetatabout it

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