Filling your tank 3/4 full

Here’s one of those bits of advice a couple of people have given me, and in these days of high petrol prices, everything to do with fuel seems important. The idea is that filling your tank all the way means that your car will use the petrol quicker than if you fill it only 3/4 full. Is there any truth to this at all, or is it like tapping the top of a soft drink can?


This is exactly contrary to what I’ve heard. No cite for this but, I was told somewhere by someone that it is better to keep your tank full at all times because with more room for gasoline to turn to gas (that’s a gaseous state) more gasoline seeps out unused.

On the flip side, with a full tank your car is carrying a little extra weight. But I think that loss of gaseous gasoline outweighs the extra weight.

That last sentence is going on my tombstone.

Sounds like wishful thinking. That might be true if fuel flow was regulated by gravity but that’s not the case. You can keep from spilling and wasting gas by not topping off though.

The advice you got is mostly nonsense. The grain of truth is in the fact that gasoline weighs about five pounds per gallon, (that’s right, gasoline is lighter than water) and you use a wee bit more fuel hauling around those extra pounds of gas. Modern gas tanks have baffles to ensure airspace at the top, so they’re never really full. They also have complex vapor recycling circuits that equalize tank pressure. The engine doesn’t care if your tank is 3/4 full or full.

I read a while back that keeping your tank topped off is good for another reason: space at the top of your tank means trapped air. Air has water vapor. Water vapor can (eventually) cause the inside of your tank to rust.

Assuming you fill your tank whenever you go to the station, I can’t see how that would be a major threat. But, if you never fill your tank past the 3/4 mark, I can see how, as the years pass, some rusting might happen. Besides, that water vapor can condense on cool mornings and get in your gas, which is just not desirable.

Thanks for all the answers - yet another Urban Legend bites the dust.

FTR, I’ve been filling my tank right up for the sake of convenience, but feeling guilty about it. Now I can do the same and feel superior :D.


You’ve forgotten the cost factor of your own time.

If I always get 3/4 of a tank when I fill and you only get 1/2 a tank, you will be making 50% more stops than me.

If you value your time at all, the wasted trips will far outweigh any possible savings.

One reason that I’ve heard a few ultra-safely concious people give is that with a full tank there is no room for air compression. So if you get in a minor accident that smashes the tank a little, it will leak and spray easier if its full. I especailly heard this after the GM truck gas tank incident.Makes sence, but I’ve never considered it worth doing.

it would shave a few pounds off the total weight of your car…which would mean less energy would be required to drive it…which means fuel savings

but the weight is so small compared to the overall weight that this is probably negligable.

you would probably save more fuel by keeping your car tuned up, the tires properly pressurized, and by driving at reasonable speeds