The maximum allowed under the 'Fixed Profit Car Scheme' is 40p per mile. If your employer has permission to use that then they can pay you up to 40p per mile without you incurring any further tax liability. They can pay more if they wish but you'd be taxed on it.
That presumes the car is your own. If it's a company car but you provide the fuel then the expenses allowable are much lower, between 11p and 19p per mile depending on the engine size.
Thoughts? It IS low. Even 40p per mile is low these days. The rate has been pegged at 40p per mile since April 2002. Petrol has virtually doubled in price since then! I think it's long overdue an increase but nobody seems to be pushing for it. 25p per mile is particularly stingy. Unless you have an especially economical small car you're probably losing money every time you do a business mile.