said by travelguy:said by talz13:It is always a gallon of gas, but due to temperature fluctuations, it may not be the same mass due to higher densities in cold temperatures, and lower densities in warmer temperatures.
Which is why Costco pumps have a label on them now stating that they sell gas by volume not energy content. Some ambulance chaser thought they could win a lawsuit because the pumps weren't temperature compensating.
Take E85. A gallon has 1/3 less energy content, so therefore it takes 1/3 more E85 to go the same distance as gas.
I always laugh when I see people filling up with E85 thinking they're paying less, and yes, I ran two tanks of the crap to confirm. My gas mileage dropped from 12 mpg to 8.