I would get a 70 tooth spur (Xenon makes a nice one) and use between a 53-60 tooth pinion depending on what fits. We typically run right around 100 mmpr on 17.5 1s so the numbers I gave should take you a little north of there depending on your tire size. If undergeared properly you may not need to detune the car any further. Even gearing to 105 or so and running the motor at neutral timing might do the trick.