I have a friend who maintains that you get better mileage if you keep your tank at least half full. She says more evaporates into the larger empty space. I think this is nuts, not to mention the impact of the added weight.
In short, your friend is wrong.
Honestly, how would keeping more gas in your car give you better gas mileage? It doesn’t make any sense.
Granted, it IS a good idea to keep a full or nearly full tank. You never know when something might happen that will force you to sit somewhere with the engine running.
Yes it might save a little fuel, but the extra weight would use far more fuel than that. Neither the vapor or the extra weight of the fuel is going to make a meaningful or measurable difference.
Not lets get to the important stuff like how many angles can dance on the head of a pin. :-)
The EPA prohibits gasoline vapors from being allowed to evaporate into the atmosphere. Any vapors making their way toward the outside world are captured by the carbon canister and injested into the engine to be burned along with the gas. Any that stays in the space as vapor just adds to the volume when you fill up again.
In short, her perception that gas vapors in the space are not burned is in error. They end up being burned.
But I agree with keeping the tank at least half full for safety reasons. Anybbody running out of gas on today’s roads is in peril not only from oncoming vehicles but also from prowling psychos. Encourage her to keep it half full.
As usual, mountainbike has explained it very clearly.
The days of evaportating gasoline are long gone, due to evaporative emissions controls, so your friend’s technical knowledge needs to be updated by a few decades.
But, as MB correctly pointed out, there are other good reasons to keep one’s gas tank at least half-full whenever possible.
Thank you. I’ll spare everyone my personal opinion of EVAP systems…