Fan power only - 800 W max?

1.32 x 100000000 Joules = 1 Gallon of Gasoline

Say you drive 1 hour = 3600 seconds, total fan energy cost = 3600x800 (joules)

So assume your car fan runs at 800 w, and you drive for one hour, it would consume 3600x800/132000000 ~=0.2 gallon of gas.

Have fun.