I wouldn't want to shim it up anyways because the motor mount has a cutout in the chassis for it to sit in. There's not enough clearance on the bottom or top for a larger spur. You could technically make it work but it would require cutting a slot in the chassis and raising the center diff top plate. I think 40-45 mph would be a good starting point with a small pinion. Having huge tall monster truck tires that also add 1-2 lbs. of rotating mass will definitely require using a different setup than you see other people using with their Tekno truggy/buggy/SCT. For all the people building one of these, I would check your estimated final speed using the 4.44 diff ratio this truck has. You may need to go down in kv depending on your setup.