How inaccurate is a speedo supposed to be

towel401

mine says 110 when i'm really only doing 101 or 103, 65 when i`m only doing 60 and that according to a gps. I know these things are supposed to display 2-3% more but is this much normal?

seanybiker

thought it was between 6-10%

Rovi

This comes up pretty regularly; here's a previous thread on the subject with links to other threads and the relevant legislation:
GPS Speed reading versus car speedometer

unkel

Indeed, Rovi.

Here's the short answer (from memory):

the speedo can show anything between the actual speed and the actual speed + 10% + 4km/h

So if the actual speed = 160 km/h the speedo should show somewhere between 160km/h and 180km/h

bikki

In my car its roughly 10%, when im doing 100 kph on my speedo, my digital reader tells me 90 kph has does my sat Nav

Alanstrainor

My speedo in my focus is out by about 10%, thought that was about standard.

macnab

I think they all tend to be bang on at town limits ie 30mph, but 10% is a good rule of thumb after that, my BMW shows 78mph when the true speed is 72mph.

Jimbo

How do you know that the Sat Nav is accurate?

EPM

to be properly sure when testing against a sat nav you'd need a 3-d unit as opposed to a 2-d one. That way you can take into account gradients etc in the road surface