a good thing to keep in mind about fuel too...

around 70º gasoline weighs about 6.2 - 6.7 lbs per gallon, depending on what source you get your info from !

in lower temps, i've read that gas weighs more.

as a general though, i go with 6.5lbs per gallon. 5 gallons being 32.5lbs if my math is right.
10 being 65lbs obviously, and so on.