Quote Originally Posted by StruanR View Post
Legally and for IVA, speedos must not under-read and must be accurate to within no more than 10% over-reading.
Most over-read 5-10% to give a safety margin to allow for tyre sizes etc. (and flatter the car's performance)
My Hawk with Alfa 12v transmission, Veglia Speedo and 29/61 R TB15 Michelins, gives spot-on speed measurements throughout the range on SatNav and phone app testing.
In other words, there is no safety margin in a speed limit, the speedo means what it says.
Also any tyre with a larger rolling circumference will cause the speedo to under-read ... illegal/IVA fail.
Similarly adverse gear ratios/final drive changes will have the same effect.
The same wheel/tyre combination and Veglia speedo, but with Ferrari 308 transmission and gearbox, speedo over-reads nearly 10%.
36mph shows 40mph, 74mph shows 80 mph.
Still legal but the other end of the scale.