Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
I've seen the Fuel Purchase Option price pro-rated when the tank is not full at check-out, but I never take the FPO.
When I check out a car that has less than a full tank and I'm not expecting to need to purchase gas before the end of the rental, I just use the trip odometer to keep track of the mileage driven, and then use a rough estimate of the car's MPG to determine how much gas to buy before I return the car. If I end up spending one or two dollars more than necessary, I don't lose sleep over it.