You know, as dorky as it sounds, there was an old role playing game/tactical game called "Car Wars" by Steve Jackson Games that was all about taking a car and turning it into your ideal "tank" and then racing it on the road against bandits and the like.

The thing is, that though the premise of the game was obviously "Mad Maxish" the way they set up the vehicle development part of the game was very realistic. It also illustrates the construction spiral as you try to balance armor, weaponry, speed, fuel consumption, agility and cost. Obviously the game mechanics might be skewed for fun, but the GURPs version might be more realistic as they have a lot of balancing calculations. Might be a good "tool" to play with before you start trying to build one for real (ignoring of course silly game mechanics like how they set up "blueprinting" your engine and the like). If nothing else it will help you set up a limit to your real life experimenting. Costs of course would have to be completely thrown out from the game as it was designed back in the 80s and 90s and the FRN is worth alot less now.