just keep your spur gear and work out the correct pinion. changing the spur by 2 teeth isn't going to make a huge difference for bashing. You are better to have a range of 4-5 pinions than a few different spur gears

Just do as James says, and find the pinion that will give you a ratio between 9.5 and 10, and then stick with it.

its a rb576spur gear the pinion will have to be about 15pinion i got told but not sure on the spur. i will try it with pinion gears first and the spur . its the same ration as the b4 and that a 81spur gear and 20 pinion gear . can i use the slipper plate of the madrat with a b4spur gear and slipper pads
thanks jimmy

thanks jimmy i will try it but i need to get a new spur gear as it worn