Speedometer Accuracy

[coopman]

Hi all - interested to hear whether your speedos read accurately. When I compare the speed shown on my speedo with that shown on my satnav/gps, the speedo reads >10% higher. My tire size is slightly non-standard, but by my calcs that shouldn't account for such a large difference.

I've been told before that most older cars slightly overstate speed, as some sort of "safety feature". That sounds like an urban myth to me though...?

[Tzantushka]

Mine is lower...

On the freeway with the speedo showing 100km/h I was being passed by cars all the time.
After that, the next time we took the E30 and my wife's XC40 (digital speedo) away for a weekend - she was the pace car and called me when she was at 100.
I needed about 105-107 on the E30 speedo to actually do 100km/h

Stock tyre size for me by the way

[paultv]

This is all normal - yes, manufacturers always set speedos to read 5% or thereabouts over the actual speed. There is a potentiometer inside the cluster AFAIK which you can tweek, drill a small hole to get external access and us a tweeker to adjust the reading to your sat nav/phone speed indication - I'm pretty sure its all not very linear, so you might show correct speed at 30mph but be over/under at 90mph.

Paul