Firstly, i'd like to thank all who was involved in creation of such atmospherical mission. Since Fort Zombie game i hasn't played anything like this. Keep it coming mates!
So, as Ihatecheaters already said, the cars are imbalanced compared to zombies. They have nothing to do with fueled-to-top car because they can't stop it. I also would like to propose some ideas regarding cars and hope they will be useful. The main idea is good old RANDOM ;) so there should not be any operational gas station at all (without options), players would have to search in any of given type of buildings (garages, farms e.t.c) in order to find any remains of fuel (canisters, barrels) to refuel a vehicle. Needless to say that location of canisters and % of their fill should be random in each start ;) I'm sure, this will make cars much harder to maintain operational.
Also there's some features i'd like to propose:
1. Make some of zombies (i.e. zombie-soldiers) armed with weapons, but make them as much inaccurate as possible (i.e. make their hands wounded)
2. If possible, make zombie-dogs (hello from "I'm a legend")