I would NOT buy anything to adjust the speedo, because of the way the Nissan ECU works out the speed of the vehicle.
In days gone by it used to be mechanical. They moved it to a gearbox output shaft sensor and when traction control/ABS entered the picture, they changed again - speed is now calculated as a 'best average' of the inputs of the ABS. In the newer vehicles, they also use this same average information to determine if one wheel is spinning faster (that's the TCS).
The ECU (on standard tyres) knows fairly accurately what speed the vehicle is doing. What's NOT accurate is the needle deflection - and that's what has to be calibrated if it's not displaying as per ADR. I've found that the ECU is quite accurate with the actual road speed (compared to GPS) - it's just a crap needle.
If you change tyre size then you'd need to adjust the ECU's figure for tyre circumference. This may or may not cause the inaccurate needle to look more accurate.
If you want to push to have it properly calibrated you can, but to be completely accurate you should have the ECU reprogrammed. I think there's a mob in the UK that do it - you have to pull your ECU out, mail it to them and wait for a few weeks.
Makes the GPS solution seem so much easier eh?