Rental car companies are under scrutiny for gas price gouging, USA Today did a story last week

A USA TODAY survey of auto rental gas prices at 13 big airports on April 25 found Hertz was charging $9.29 a gallon at all 13. Dollar and Thrifty were charging $8.99 a gallon at two.

At $9.29 a gallon, Hertz customers renting a Ford Club Wagon, which has a 35-gallon fuel tank, would owe Hertz $325.15 for gas if they returned the wagon with a nearly empty tank and hadn't prepaid for the gas.

Renters who pay ahead of time for a tank of gas from Hertz or another car rental company, though, may find the per-gallon price cheaper than at many local gas stations.

Of 102 prepay prices charged by the eight big car rental companies at 13 airports on April 25, 54 were cheaper than the average price at local gas stations that day. Forty-three prices charged for prepay gasoline were higher, USA TODAY's analysis of rental company prices and the AAA auto club's gas-station data found.

Most or all rental companies' prepay prices were less than the average at local gas stations

Though renters who prepay may pay less per gallon than at local gas stations, it's questionable whether they save money by doing so.

Prepayment is for a full tank of gas and can benefit renters who return vehicles with a nearly empty gas tank. But many renters who pay ahead return vehicles with a substantial amount of gas in the tank and are not credited for it.
http://travel.usatoday.com/news/story/2011/05/Rental-car-gas-prices-hit-more-than-9-a-gallon/46778642/1

In February of this year I checked the price of gas at a couple rental car companies here in San Diego: http://justacarguy.blogspot.com/2011/02/rental-car-fuel-charges-as-of-last-week.html