speedo drive

With the new speedo fitted (nothing else has changed) I now have a large over read error in speed reading when matched to my GPS whereas with the old one it was accurate and matched my gps.

I have lt77 gearbox, lt203 1.2 (disco) transfer box, standard 110 diffs (Salisbury) and 265/75/16 tyres. I am currently running a blue speedo drive which I believe is correct for this setup. from a bit of googleing I have found the below:


frc3310 blue - 20 teeth - 7.50,235/85 or 255/85 tyres
frc3311 green - 21 teeth
frc3312 yellow - 22 teeth - 205 x 16 tyres
frc9339 Black - 23 teeth
frc3313 red - 24 teeth

Please someone check my logic here: as my speedo is over reading I am assuming I want the drive to spin slower to reduce the speed reading. In order for the cog to spin slower it needs more teeth

On the above assumption I have created the below table using the tooth ration listed above:



With the ratios above I then created the predicted speeds for each drive based on my current observations and calculated speed. (I need to check my GPS in another vehicle for accuracy before proceeding further)

Based on this it looks like I need a yellow speedo drive but I know that is definitely not correct for my vehicle. Unfortunately the price of the speedo drives has increased so i do not want to just play trial and error. Can anyone validate the above looks sensible and that I do indeed want a different drive to the blue one I have at the moment?

GPS speedo accuracy is confirmed as being consistent, same app on two phones gives the same reading.
Below are the readings I got from the TD5 and the 110. TD5 speedo matches the GPS speedo.



This give the 110 an average overread of 15%. so I think I want a 22 or 23 tooth speedo drive?

Well as all of the tables of figures and maths clearly scared people away I have ordered a yellow cog and also have a black cog I got given by a friend so I will see which of these gives me the most accuracy without under reading on the speedo.

My question now is how do you swap the cogs over in the drive and do I need change any seals/o-rings etc while I am in there, I have not yet pulled the speedo drive so it may be very obvious once I have it in front of me.

Black cogs have a larger end where the cable goes. Unsure of the yellow.

To change drive you turn slightly and pull out.
Long nose pliers in the drive end and tap if stuck

Hicap phill said:

To change drive you turn slightly and pull out.
Long nose pliers in the drive end and tap if stuck

Click to expand...

Many thanks. My understanding is yellow is a direct swap as it is what was fitted to 90s as they ran 205’s