Ive got some friends who always offer me money if we go out for petrol

. I also have done a car share arrangement for people for work and some of those offer money when they cant reciprocate.
What do you think the right amount is to give to a mate

I know the tax man allows something like 40p per mile but this seems a bit steep to give to a mate or would you measure the exact amount of fuel used and split it 2 ways.