Evening rush hour on the 110 Freeway in downtown Los Angeles; faced with growing shortfalls in highway funding as drivers give up gas-guzzling cars, California officials are testing the idea of a mileage fee to replace the state tax of 36 cents per gallon. Evening rush hour on the 110 Freeway in downtown Los Angeles; faced with growing shortfalls in highway funding as drivers give up gas-guzzling cars, California officials are testing the idea of a mileage fee to replace the state tax of 36 cents per gallon.