on an R32 ( cable driven speedo) the speed is taken from the same place as the main speedo, (the RSM always shows the excat same speed as the speedo needle, not the car speed) so to calibrate the RSM you would have to get the main speedo altered.
(I just fitted a kmph - mph converter on my car, and the RSM still reads the same speed as the speedo... ie, at 60mph the RSM shows 60kmph not 100kmph)
If your car is an R33 you will have the electronic speedo and im not sure if you can change that...
does the RSM show a different speed then to the speedo?? or do you know that the speed is out on both?
Have you set the RSM to the tyre size and profile that you are using? The RSM comes with a factory setting which is equal to 16" wheels, if you are using bigger or smaller, the ratio should be adjusted accordingly.
As the tyres wear the KPH/MPH reading will alter slightly anyway.
Hope it helps,
not too sure on the exact method of changing the tyre size, but off the top of my head the option is in the 'etc' 'car select' menu, unfortunately the thing cant be altered to read mph mine shows the same speed as the speedo, but still has kmh written on the display...
They will show you how to adjust for the size of tyre you have.
In ETC there is a wheel tyre setting of 100% which is correct for 16" wheels. I calculated for the 18" Nismo GT1s that it would work out at 108%.
Follow the guide and calculate your ratio or else just try increasing it till it is the same as your speedo.