Chuck has provided the correct maximum amounts that you can be paid as 'expenses' without having to pay tax on the payments.
In practice they're far more than most people receive. One firm I work for increased their rates last year, from 17p per mile to 19p per mile. Another firm that occasionally employs me also increased their rates last year from 20p per mile to 25p per mile. However the first firm also pays me 'travelling time' whereas the second one doesn't. That's an important consideration when determining what is 'fair'.
At 40mpg, and with petrol around 90p per litre, roughly 10p per mile will cover the petrol costs. If you're driving a top-of-the range BMW, the additional insurance costs for business use and the increased depreciation through the extra mileage will add quite a bit to that. If, like me, you're driving an ancient Mondeo which you bought a couple of years ago for �350, with only 3rd party insurance, business usage adds almost nothing to the 'cost per mile' of running the car.
20p per mile probably covers most people's additional expenses through using their own vehicle, but is still generally regarded as being rather stingy.
30p per mile is quite generous and should (more than) cover most people's additional expenses.
If I was a shareholder in a company which was paying 40p per mile, I'd be asking what the bloody hell the directors thought they were playing at, and seeking to get them removed from office.
Chris