Pimp Car achievement in Zombie Driver Ultimate Edition

First off, you need to upgrade the Armor, Ramming and Speed to the maximum.

This can be done on any vehicle, but I recommend the bus (was very useful for missions where you needed to save more than 4-6 people since it holds up to 48. I would just keep driving until I picked up everyone and brought them all back at once - instead of making separate runs in a smaller vehicle).

You will need to get to mission 20 to unlock all upgrade slots so don't try to do this until you're about 2/3 the way through the game. You should have plenty of money to unlock all weapon upgrades (these carry over from vehicle to vehicle) and fully upgrade one vehicle without worrying about the secondary objectives.