Accurate? That's a tough standard. They don't take the car from you, fill it, and use the volume from the meter to the nearest tenth of a gallon.

Fair? The mechanism if one hasn't purchased gas appears to be distance traveled/EPA rated city mileage * contract gas price per gallon. If one drives 30 miles in an Impala that means you're going to buy about 1.5 gallons, or around $12 (unless you're using a corp. code with a market-based fuel cost per gallon).

Rational people will recognize there's a value in not needing to stop (the Gas Buddy suggestion plus GPS nav makes things easier). Whether that's worth the difference between National's fuel charge and do-it-yourself is up to you. I won't risk missing a flight to put gas in a rental car, but unless I'm running late due to a long-winded client, I don't try to cut it close, either.