Miles per gallon in quoted figures is generally done very gingerly in a wind tunnel with a perfectl tuned car, an impecible driver, and perfect conditions. In reality stop/start motoring, with busses pulling out in front of you, traffic lights and other factors will all afffect the figure.
Why do you not suggest they pay a fixed rate per mile?I believe the Inland revenue figure is 35 or 40 pence per mile, seen by them as a reasonable amount.
Suppose you get a puncture whilst on company business, who will cough up the �50 or so, you or them?
Perhaps a tax adviser type person could advise better