40p is the maximum you can pay as a tax-free expense for the use of the employee's own car - but only for the first 10,000 business miles in a given year; any mileage in excess of that can be claimed at a maximum of 25p per mile.
At a firm I used to work for, they based the mileage rate on the engine size of the car (3 bands with more cash per mile for bigger engine sizes); I was lucky at the time, as I drove a 1.8 litre Peugeot 205 diesel back then, so claimed the highest rate of mileage expenses for a car that did 50 miles per gallon (as opposed to a gas guzzler doing perhaps half that lol)...