Just to make sure I understand. You want to change the speed of vehicles to make them go faster? If so see below.
Go into the base vehicle class. I'm using TigerTank.uc for this example and look in the deafult properties. Find the below set of properties.
GearRatios(0)=-0.200000
GearRatios(1)=0.200000
GearRatios(2)=0.350000
GearRatios(3)=0.550000
GearRatios(4)=0.720000
TransRatio=0.09;
The two properties work together. Basically, to make a vehicle go faster you can increase the TransRatio. You can also change the GearRatios to increase acceleration. I think you can also impact the top end speed by adjusting GearRatios(4) in this example.
Don't quote on this, but I'm pretty sure GearRatios(0) is the reverse gear and the 1-4 are the forward gears. You can compare values between some of the faster and slower vehicles to give you an idea of how to change the values.