The lower turn motors need lower gearing too (smaller pinion/bigger spur). Other wise the additional amp load will tend to make it run real hot if not burn something up.
Done correctly the higher motor speed will make up for the gearing difference and the gearing will bring the torque at the wheels back up, and then some.
For the brushless stuff Novak has a gearing guide that will give you a safe start. For brushed check your vehicle manual or ask around.
Welcome to the insanity, it is a lot of fun.