Well, the tires on my GTP are closer to 4.25", which will drop the speed to around 35mph. You can use a 14T pinion to get back up to ~37mph. I don't think you'll have any problems with a 15T (~40mph) or maybe even a 16T pinion (~42mph), if the vehicle is kept light. But, a 14T pinion is a good place to start.
Well i ordered my GTP today. It will be here friday . I plan on using the RC-monster motor mount with a MEDUSA 70mm 2300kv motor on 4s flightpower lipo with a MM esc.