There was a thread on this subject running last year. Apparently Nissan set their speedos at the lowest possible (legal) setting. Somthing like a reduction of 10% plus 3 mph, (which ties in whi my own experience of the speedo showing 70 and the GPS showing 60) although I can't quite remember the exact figures. It apparently gives Nissan a big comfort zone from any possible litigation regarding speeding fines caused by speedo inaccuracies.
If i remember right Shark1e was hoping to have a play with his and possibly produce a workshop on adjusting the speedo closer to the actual road speed, but with the twists and turns of the last 12 months I not surprised that he never got arround to it.
I guess we will just have to live with it, still at least we are not going to get done for speeding.
I am still concearned however if the same deviation affects the mileometer, as even the 10% error puts lot of miles on an older car. I have seen on the net specialist companies who are offereing to calculate and adjust milometers to the correct reading, but even if you did this you would still have to declare it at sale as being non-genuine mileage so it does not seem to be worth while.
Cheers
Rincewind