W124 Speedo Problems

[NIPhil]

My W124 E220 is an import and the speedo was in KMH only so it wouldn't pass the Northern Ireland MOT.

http://i204.photobucket.com/albums/bb215/PhilGSA/W124Speedos_zpsf19129dd.jpg
I bought a 160mph speedo from a member on here and whilst driving to the MOT centre noticed that it was reading 50mph when I was only doing 40mph (Via Sat nav).
I then got a 140mph speedo fitted and still it reads 50mph when I'm only doing 40mph.
http://i204.photobucket.com/albums/bb215/PhilGSA/140mphspeedo_zps59a2bc3d.jpg
Then I had the speedo drive/worm drive/gubbins replaced in the manual gearbox using parts from a UK gearbox.
On a test run...............still reading 50mph when I'm only doing 40mph

How do I solve this???????????

 

[Stratman]

Is it a fixed 10 mph error throughout the range? If so, prise the pointer off the spindle and replace it as required.

 

[NIPhil]

Is it a fixed 10 mph error throughout the range?

No - the Odometer has clocked up 475 miles when I've only covered 379

 

[Stratman]

The speedo and drive gear are mismatched. You need to discover the 'turns per mile' figure of the speedos. Way back it used to be written somewhere on the front but I can't see it on either of yours. It may have been just a Smiths thing, but without knowing the numbers you'll be guessing at the drive gears. Have a close look for a number round about 1000, give or take a fairly big amount, somewhere on the units. If you find them, simple maths will tell you the drive gears you need.
If you get one from a car with the same spec (engine, transmission and wheels/tyres) as yours you will probably be OK

Google for "speedo turns per mile" and "vdo speedo turns per mile"

 

[balge]

I believe the speedo has to match the diff to read accurately? (wheels too, obviously)
cheers

 

[carat 3.6]

The speedo is matched to the diff on these cars, so you need to find out what ratio diff you have. It will be stamped on the rear of the diff housing, to the lower right of the rear cover plate.

I'm going to guess that your car has a 3.67 diff ratio, so you need to find a speedo from a car with the same ratio.

 

[300CE]

I'm pretty sure Practical Classics magazine recommend this firm for speedo calibration & they can do this for you - might be worth giving them a call to see if they can help beforehand:

Calibration

 

[NIPhil]

Gents, thank you for the suggestions - I'll get on to it.