I take it your car is a diesel.
DO NOT START THE ENGINE WITH A 50/50 MIX!!
You could just fill the tank with diesel and run it, topping it up every 40 miles or so to dilute the mix as much as possible. Do not over-rev or overload the engine.
Some diesel mechanics put a little petrol in the diesel from time to time to 'clean the system'.
However, if it was my car, and given the amount of petrol involved, I'd be siphoning/pumping as much as possible of the mixed fuel out then filling up with diesel.
You should be able to mix the contaminated fuel in gradually, but it might be safer to get rid of it. Your local council can tell you where to dispose of it safely, or if you know anyone with an HGV tip it in their tank!
bbc333, May 2009
