What vehicles?
For land you probably want to look for these lines, though I am not familiar with land.
ObjectTemplate.setTorque 350
ObjectTemplate.setDifferential 4
For helicopters its more tricky to do it right,
ObjectTemplate.setTorque 325
ObjectTemplate.setDifferential 175
ObjectTemplate.horizontalSpeedMagnifier 2.1
ObjectTemplate.maxAngleOfAttack 12
ObjectTemplate.attackSpeed 21
will all have an effect but you just have to play around and balance it because otherwise you can end up with insane jet intercepting UFOs or something like helis in PR where it will insta accelerate into the ground.
I am sure there are much more variables for the land vehicles though, there are for the helis which you have to take into account. Gravity, drag and many things can have an effect on the handling. Just increasing the speed simply won´t do unless its a very small change.