I would ask at your local track what the preferred overal gear ratio is for the motors you use.
Then see how possible it is for you to get that ratio with a combination of spur gears and pinion gears, at the same time taking into consideration the problem you have.
As a last resort.... Is it possible to remove some metal from the motor mount holes, so you can gear it differently? Although, you really shouldnt have to do this.
Fusion Hobbies / www.horshamrc.org
The wife stops me being sane and the r/c stops me from going insane....