Some *ARE* faster than others VexD and some are slower ... like gulfs apart ... but I'm hoping to be able to hit this from two angles.
For starters I've got them all within the same ballpark for power-weight ratio with HP$ upgrades. Now I know that this isn't everything, so what I then did was take all of them (modded) onto 3 different tracks (Jump_Ace was invaluable here, because he is able to run consistent times and gave me some good benchmarks).
With these 3 sets of times I can see which are too fast and which are too slow ... although perhaps surprisingly the power-weight ratio thing has brought most of the field (all bar maybe 4) into the same sort of vicinity.
The ones that are two fast (2 of them) will probably get less for HP upgrades and those that are too slow (2-3 of them) may get a little more.
On top of this, I'm handing out varying amounts of non-HP upgrade money, which I hope will bring the gaps even closer.
I share your concerns, but this is why I've spent the last month or so buying every car, figuring out the HP gained from every upgrade, and then taking them all to the track. I really think that while there will be some differences, I can get all the cars to be competitive and on a fairly even playing field.
*fingers crossed*