Correct Speedo for 3.9 ratio diff

[jaguar68]

Hi Guys,

Not sure wether this is the right forum area but can anyone tell me the correct TPM for the 3.9 ratio diff and a likely source?

Cheers

John

[bmcecosse]

I can tell you that 1248 is absolutely correct for a 3.7 final drive - and 155 X 14 tyres. So - for a 3.9 you need to look for a speedo something around 1315 tpm. Not sure if you will find one - there are a range of Mini speedos that fit right in the Minor - you will just need to ask every seller on ebay what the tpm is on the speedo they are selling!!

[jaguar68]

What would be the result of using a higher TPM speedo, for example 1376 instead of 1315?

Cheers

John

[bmcecosse]

It will under read by ~ 4% In other words - you will be going 4% faster than the speedo reads. Not too bad really.

[mike.perry]

i run a 3.9 diff on 175 x 13 tyres which have a very slightly larger rolling radius than 155 x 14s, + 6mm, and I still use a 1408 TPM speedo which I have checked against my Sat Nav (me driving and the passenger checking) and the speedo was within +/- 1mph throughout the range 20 -80 mph. It just shows how inaccurate the speedos are on the standard diff.
So before you start looking for a new speedo check the original, you may save yourself some money.
There is a speedo chart on the Series MM website http://seriesmm.mmoc.org.uk Car Ident.

[jaguar68]

I've just bought a Mark 3(?) Mini speedo, the one with the concentric circles on the face. It was only £11, so nothing really lost. The car will be running (hopefully) 155 x14 or 165 x 14 tyres or whatever fits on the LP917 LCV rims.

Regards

John

[jaguar68]

My new speedo arrived today, luckily, it's a 1376 TPM ratio, so hopefully shouldn't cause too many problems.

Regards

John