Same thing happened to MPs a couple of years ago.
Parliamentary allowances: Motor Mileage Allowance
2004/05 mileage rate: up to 20,000 miles per year at the rate of 57.7p per mile, and over 20,000 at 26.6p per mile. (For 2005/06 this rate has been reduced to a rate of 40p and 25p per mile).