Speedo Readings considerably out...

I have just collected (yesterday) an extremely well presented (magazine featured) 1983 Mk1 GTi and had an enjoyable 100mile journey home…The only 'issue' is that comparing Satnav speed to the speedometer, the discrepancy I consider excessive: -

At satnav speed 20mph, speedo reads approx 30mph
At satnav speed 30mph, speedo reads approx 42mph
At satnav speed 70mph, speedo reads approx 84mph
However, the recorded mileage is only around 2% high over the distance… This I consider within totally acceptable parameters


Thoughts and any potential solution would be greatly appreciated.

Many thanks
Ian

Original wheels and tyre size?

No - they are 195/50/R15… I'm sure the mileage would be out also even though they're not standard. Error in speedo is considerable…

Has the cog on the end of the speedo cable, gearbox end been changed and what colour is it?

Has the instruments been taken apart, there is a line you have to calibrate the needle on, lift it (very carfully) over the stop and it should line up.

Normal for VW's to be 5 mph out compared to a Sat Nav, 50 mph on sat nav is 55 mph in the car, my wifes 2006 VW touran reads this and a T5 of my dads is the same.   

Sat nav, while accurate is also a little iffy.  It depends on which Sat you are locked into at the time of the measurement.  There are also variances in them to prevent them being used as guidance systems for ballistic missiles.  (I know more than some on this.)

Larger tyres will cause Speedo's to be off, and the larger the tyre the greater the offage.  The standard oem tyre on the Golf was a 155sr13 originally.

There is also a number stamped on the back of the Speedo which relates to the car, I think it is 1578 or 1478, been a while since I had a cluster apart.
This along with the Diameter of the CV as in 100mm or 90mm along with the gearing of the tranny can add up to a wide variance of speeds.

Kind of like if I am coasting down hill in my Van I get 92mpg, while on a flat I get 22.

Here is a link to a size comparison of readings that you should get by tyre size.
Speedometer Calibration

An a link to the Gearing differences between the transmission gears.

http://www.cabby-info.com/transmission.htm#Gears

Ratio's of most transmissions.
Transmission Ratios

I think that if you probably change out your drive gear you may improve the differences.


 

Thanks for the comments. My main 'concern' is that the speedo is 'well adrift' of actual, but mileage records is more or less spot on ( approx +2%). I've had cars where the speedo was out by 7% but this was also on recorded mileage.

Cheers

Mine is exactly the same as yours think our Speedos have been taken apart and the needle put back without it being pryed over the stop needle, I starting to live with mine and use a Sat nav sometimes as a precaution  and also see the flashing speedo signs on roadsroads and the Sat nav matches them exactly, I would take it apart and put it right but I'm glad it's working as my last one messed me up as the Mylar was brittal, lol let me now how you get on I might have the courage to do mine In the future

BAZ-GTI said

Mine is exactly the same as yours think our Speedos have been taken apart and the needle put back without it being pryed over the stop needle, I starting to live with mine and use a Sat nav sometimes as a precaution  and also see the flashing speedo signs on roadsroads and the Sat nav matches them exactly, I would take it apart and put it right but I'm glad it's working as my last one messed me up as the Mylar was brittal, lol let me now how you get on I might have the courage to do mine In the future

Why not you do yours first and let me know!!!  

Cheers
Ian