Same way you make a good level: with good ratios.

Pick a car with certain gravity and mass stats....tweak it till it looks good...now use this car as the benchmark and modify each car accordingly.

Example: Ford mass = 1000 kg Chevy mass = 2000 kg.

There is a 2 to 1 ratio in chevy to ford mass.

Now you tweak everything and find that in 3DGS, a Ford moves juuuussssstttt the way you want it, but you had to set ford.mass=25764;. Now, what is the chevy mass? Since it's a 2-1 ratio, mulitply that by 2 and set chevy.mass=51528.

This will give you the correct scale just about every time. If it doesn't, then you can perturb around these first order values to find the correct "feel" to your motion.

(PS: Even if what I said was untrue, a "real" car simulation is still a very hard thing to do )