First off most cars build after the 1980's don't have a carburetor and generally use fuel injection. Also, to much water in the tank can easily lead to a blown head gasket among other things.

Second most cars don't cost 40k, but if your talking about a say a bugatti veyron it's not that hard to do 40k of damage. If you want to compare it to the cost of the car then for an older car having a few thousand dollar repair bill can easily cost more than the car is worth.