Speedos must not under-read, and are legal up to 10% over-reading. So an indicated 77 for an actual 70 is ok. 78, not so, nor 69.
GPS speed is not totally reliable, although it should be so in a straight line. If you have a local measured mile you probably can improve on the accuracy (ask local traffic or weights and measures - mine are very helpful to folk who want to measure things).
My vintage speedo reads 64/63 for 60mph, but that's the way I calibrated it. PM me if you want the method, but it's only applicable to mechanical speedos so no use for 5 speeders.
