Car/Bike speedo's have to read within a band of -10% to +0% so, if your speedo says you're doing 70mph, then you could actually be doing anything from 70 down to 63mph. I had my bike's speedo "compared" with a guy I know who's speedo is "calibrated" for his work (ahem) and it was spot on at an indicated 70mph but I needed to be doing an indicated 33mph to be doing an actual 30! That was on well worn Diablo's. I did a simple check with him again when I had new tyres fitted and it was spot on all the way from 20mph up to 70. This is because a new tyre has a larger rolling circumference than a worn one and centrifugal force has a greater effect on a worn tyre than a new one (obvious when you think about it) so you travel further for each turn of the wheel/engine. If a motorcycle is heavily banked over round a fast corner then you'll be using a smaller diameter part of the tyre and the rolling circumference will be greatly reduced so you'll actually be going LOTS slower than your speedo may indicate!
From my limited experience it seems that stabilised radar/laser targetting devices in the hands of a trained operative are generally regarded as "absolute" measuring devices and it is up to the operator to use their judgement on the spot regarding the prevailing conditions to determine the actual course of events once a certain speed is indicated on the recording instrument, using the (very old, and some say out of touch with modern technology) ACPO guidelines as a yardstick, so you
may get away with 79mph in some 70 limits and get done for much less in others.
Not that I know anything about the subject of course
