Whats included in a car allowance?

I'm in the process of deciding whether to accept a company car or a car allowance but I'm wondering how the allowance is calculated. The company is telling me that the car allowance would be a full and final contribution to running the car and that I wouldn't be able to claim expences based on mileage on top. However there must be a difference if I travel 5,000 miles or 25,000 miles. Do they assume a certain amount of miles and can I argue that anything in excess of this is claimable? Also does anyone have any typical allowance figures for a Golf?
Any advice gratefully received!

In my case, I get a car allowance, and a mileage rate for every mile travelled, though the mileage rate is much lower than that for staff who don't get a car allowance.

Thanks for that. I received clarification on the offer so it now includes an allowance plus a fuel card. That puts my mind at ease on the variable mileage issue although there's still issues of greater wear and tear, etc. but the allowance is reasonable so it should cover those as well.