Talking of US fuel taxes, how much is a gallon of petrol over there ATM?

In the UK we're not allowed gallons of petrol any longer but for comparison it works out something like this:

1 liter = $1.19USD (£0.74GBP)
1 US Gallon = 3.78l = $4.50USD (£2.80GBP)

Incidentally, does anyone know the reason for having two different gallon capacities (US & Imperial)?