Series 2A 1968 SWB 2.25 diesel speedo under-reads

My speedo and mileometer are under-reading by about 23%. I’ve discovered that I have 3.54:1 ‘90’ diffs fitted front and rear. This is also bourne out by there being no oil filler plug in either the rear axle casing or diff housing. As far as I know I have a standard 2A gearbox and transfer box. Tyres are 205/80 x16. I’m surprisingly happy with the resulting gear ratios but I’d like to have an accurate speedo unit. I have the standard 1536 tpm version but if I fitted a 1408 tpm unit from a 109, this would only improve readings by about 9%. Am I forced to get my existing speedo unit recalibrated or is there another option? I reckon I need about a 1183 tpm setting.

try speedograph Richfield ltd they can recalibrate your speedo. if you look on their website it explains what you need to do. I have to send one of mine to get it done for same reason. costs about £75

Thanks minecab. I’ve checked the site and it sounds the right place to get the recalibration done. I had hoped there would be different toothed mechanisms to try in the series transfer box But it seems not.
Edward

Given the age some of the problem may well be in the unit itself.

Thanks tottot
That is what I thought at first. To check I compared an identical route and distance taken with the Series Landy and with my Disco 3. The 23% difference in mileometer readings agreed with the speedometer reading difference on the Series Landy against my son’s GPS. My understanding is that the mileometer is a direct mechanical drive from the transfer box and should be pretty accurate as standard tyres are fitted. My next job is to jack up a rear and a front wheel to actually confirm what diffs are fitted. If the diffs turn out to be the standard ones, I could try to obtain a replacement speedometer unit. The funny thing is that the gear ratios when driving appear about right and I think I would notice if there was a 23% higher gearing present.

They work well enough if standard set up, one sign of problems is the MPH needle jumping about a lot at certain speeds, but this can also be a cable drive problem.

The drive gear for the speedo is only nipped between the out put flange and the bearing if the nut is slack speedo drive can slip and under read / jump about.

Thanks Tottot and Blackburn. The needle is not jumping about but I hadn’t thought about the possibility of the drive slipping. I will have to investigate further.

Blackburn said:

The drive gear for the speedo is only nipped between the out put flange and the bearing if the nut is slack speedo drive can slip and under read / jump about.

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If it is loose enough it doesn't work at all. When I bought mine the speedo did not work and the PO supplied a new genuine speedo drive pinion to rectify the problem. When I investigated it was loose, tightened it up and it has worked ever since, the new pinion is still sat in its box in the garage.

Speedo drive pinions are worth a lot of money now have a check on sale prices , was going to give my spare away but seen price and kept it.
Wonder what affect going on and off load has on a slack output flange?
Will it thrust back and forth ie picking up the speedo drive when it does!