Hi,
Since iI had purchased the car I had noticed that the spedometer has a false reading. At first I belived that was showing in miles, but later I checked and I was puzled.
Cheking my speed with GPS: At 52km/h it reads 40, at 101 km/h it reads 80. The guy who sold me the car told me something about a coghweel not fiting, but that`s it.
My question is: how does the spedo measures the speed? Does it measures mechanicaly, with cable, or electronicaly?
Thanks

 

I am no XJ6 expert; but I think you will find the speedo is driven from the gearbox by a special gearwheel, that comes in a range of sizes. Normally these can be changed for the correct size with the gearbox in place. As it is reading about 25% slow (that is the speedo reading needs to be increased by 25% to get the actual speed) I think - repeat think - you need a cog with 25% fewer teeth. As the gearbox internal speedo drive gear is the same, fewer teeth on the replaceable gear means the speedo drive will turn more quickly - I think!
This an example from a Series 2 XJ6, probably not correct for you car; but it gives you an idea of the sort of thing you need.
https://www.ebay.co.uk/c/1315872845

 

Series 1 used a matched speedo head for each gear ratio. The number on the speedo face should match the ratio in the book. The odometer will also be wrong...

Look for a metal tag on the rear end or measure the number of driveshaft turns for one turn of rear tire.

 

This is the Speedometer Drive for an XJ6 Series 1:







The square peg (arrowed in red) engages with the gearbox output. it locks in position with the fork and is tightened with the knurled wheel. A flexible cable goes from the threaded section arrowed in blue to the speedometer.

I don't have my XJ6 Parts Catalogue any longer but recall this angle drive was available in a number of different options to suit the final drive ratios. I only remember because I had one fail on my 1972 XJ6.

Graham

 

Maybe this page 25-62 from the parts manual will help you locate the needed parts...

 

Thaks guys!