A chassis dyno will be able to tell you what percentage speedo error you may have fairly accurately- get the operator to set the roller speed to 100km/h and see what your speedo reads, then again at 150 or so. You can then work out what % it reads over or under the actual speed.
Somewhere on this site, you should be able to find out various ratios, but some i know are-
R32 GT-R (GTS-t too?) - 4.11:1
GTS4 - 4.375.