31-10-15, 06:34 PM
Stick a satnav on, go for a run at 50mph and 100mph, note difference betwen speedo and satnav. Then just allow for the difference , costs nothing 
At 100mph on my satnav my speedo reads 97, rather be under than over.
At 100mph on my satnav my speedo reads 97, rather be under than over.