I was wondering what kind of pinion gear to buy what make I will be using them in my T4 and my sons XXX T are they interchangable or will I need different ones for each truck.

make sure the pitch is the same as your spurgear by what ever you can aford they sell for 2.50 up to 5.00 apeace you will be ok.

Off-road cars and trucks use 48 pitch gears. Any major brand would be fine.

The T4 and B4 both have 48 pitch spur gears....I use Robinson Racing Pinions as well as PRS. Just make what ever brand you purchase they are 48 pitch.

as long as you match the pitch, any pinion gears are interchangeable