A decent game :)
I will be honest here, this game is good.
A few tips:
1. As you are buying better guns, you focus on using the most expensive one, probably because it is better than the other ones you have. Try reloading all guns when you're not occupied by zombies, and then after your primary gun runs out of bullets, switch to your other guns and use up those bullets, it will help a bit on lowering the zombies, and then switch back to your primary gun and reload and focus on that gun afterwards. Then, on the next round, while you're trying to repair your car as much as you can, reload your guns again in the process and repeat.
2. You probably know that standing on top of your car gives you better survivability, and you can use this to wipe out a lot of zombies before you have to set foot on ground, but, in my opinion, it would be best to not go on top of the car on every stage, and instead, move around and shoot the zombies, because this will hasten your repairing time for your car, and there is a very low chance you will die if you have points on hp regeneration.
That is all I have to share after playing the game :).