Speedo Accuracy...


Registered User
Prompted by the thread about the M42, found an interesting characteristic in my Focus the other day. If you reset the average speed on the computer whilst holding a certain speed, it resets to match the GPS speed displayed on my Snooper, not the speed displayed on my speedo.

Does anyone have any ideas why? If the computer knows i'm going at a certain speed (as shown by the GPS), why show a different (slower) speed on the speedo?


Masa'warty 3200... Talk To Me!
I take it the Speedo is not Digital?

All analogue speedo's seem to be in accurate, I am not sure why all makes of car i have ever had or friends of had have all been in accurate. They tend ti under read up to 30 then after that they over read.

Dunno if it is to keep actual speeds down. So that drivers are going at there indcated say 70 mph when they are really only doing 67 mph. GPS is deffo more accurate than your standard speedo.

Can anyone explain how the speedo reading is produced is it direct from the gear box or something. That may explain it as the gear box should i think, spin quciker than what the wheels are doing. Maybe this produces the inaccuracy, as at ceratin speeds, the way the figure is produced is accurate but after a certain point it becomes increasinly in accurate after this point.

Does this make any sense as i am half asleep at my desk at work. /ubbthreads/images/graemlins/sleep.gif


Active Member
It's an EU directive I think so that drivers cannot claim their speedo was under reading.
All analogue speedos over read by anything up to about 7%. The one in our B6 is atrocious, it reads 92 when actually doing 85.
It'll be built into the analogue meter, which has to convert the digital info to an analogue speedo reading


hud at ye bam
If you enter the codes and get the digital speed display on the climate control, would this be the same as the analogue display?

Haven't tried it myself.


Active Member
It's in km's so not very useful


Registered User

You also need to bear in mind that different brands of tyre even though the same size i.e. 225/40/18 actually have a different circumference.

They also need to factor in tyre wear.

Because of these things speedo's are inaccurate.