7 replies to this topic

[JamieBayliss]

Hi, I have been researching all afternoon but seem to be confusing myself even more......

 

I am building a new dash and would like a Single Speedo and single Rev counter instead of the 3 clock dash which is currently in my mini.

 

I have a 1985 Mini Mayfair (I beleive with a standard 3.105 FD) with 165/60/12 tyres.

 

The 3 clocks I have numbers '287' and 'Lucas Code IP4803/14c' on the rear.

 

I think I know how to work the wiring out but was confused as to which mini/mg or any smiths single speedos would fit to read the correct mph.

 

Thanks for your help.

[JamieBayliss]

Anyone please.

[surfblue63]

There's a good amount of info here on were to find the TPM of your current clock.

 

http://www.guess-wor...Tech/speedo.htm

 

 

This article shows how to find your TPM by means of pushing your car and measuring the rotations of the cable.

 

http://www.dbraun99....meter cable.pdf

 

 

MG cars have speedos with TPMs which range from 1000 to 1480, so to find the right one you need to match the TPM of your current set up. It is possible to change the TPM by changing the drive in the gearbox, Guessworks have a calculation program on their site for this.

 

http://www.guess-wor.../Tech/ratio.htm

Edited by surfblue63, 20 May 2014 - 04:19 PM.

[JamieBayliss]

Thats brilliant help! Thanks very much. Much appreciated. 

[colinu]

Another useful site is www.tomsminisite.co.uk/speedo_calculator.php (I used that and Guessworks' a few days ago as a start to figure out my TPM and hence what pinion/worm I needed to drive my 1248 speedometer correctly).
Assuming you do have a 3.105 diff (and 7/16 speedo drive ration) then Tom's calculator shows your speedo cable will be turning at 1384 TMP.
 
HOWEVER, if you're not sure what diff and speedo drive gearing you have then here's how I calculated the TPM of my speedo cable...
 

• Pop/unscrew the speedo cable off the back of the speedometer and poke it through the dash so that you can see its end
• Put some form of marker on the square inner cable so you (or easier still an assistant) can count each revolution (I used a cable tie, but a bit of blue tack with a peak on the side would work too)
• Put the handbrake on (safety first!), jack up only ONE front road wheel, put the gearbox in neutral
• Now start turning the road wheel... you count the the number of turns of the road wheel, while your glamorous assistant counts the turns of the speedo cable
• For me 14 1/8 (14.125) turns of the road wheel resulted 10 turns of the speedo cable (good enough to give me a pretty accurate result, but the more turns you use then the more accurate your calculation/result will be)
• Now the maths...
• Like you I have 165/60 R12 tyres which take 1019 turns to cover a mile
• With only one wheel in the air the diff acts like a divider... so in reality you need to multiply the speedo cable turns by 2 (e.g. for me... 10 x 2 = 20 )
• Next you want to calculate the overall ratio of speedo cables turns to real road wheel turns (e.g. for me... 20 speedo turns / 14.125 = 1.416 )
• Now to get the speedo cable TPM you multiple the wheel turns needed to cover a mile by that ratio (e.g. for me... 1019 x 1.416  = 1442.9 TPM)

 
So the formula is...       Cable TPM =  (Turns needed to cover 1 mile) x (speedo cable turns x 2) / (road wheel turns)
 
OK so obviously my own calculation of 1442.9 TPM has a small error as that ratio doesn't exist for any wheel/diff/speedo drive combination, but the above referenced sites (Tom's and Guessworks) show the closest to it is 1445 TPM. None of the standard Mini speedometers use that ratio hence for my Mini with 3.44 diff I figured out I need to change from a 7/17 to a 6/17 speedo pinion/worm combination to correctly drive the 1248 speedo I've fitted :)
 
 
   Cheers,
   Colin.

Edited by colinu, 23 May 2014 - 03:50 AM.

[dklawson]

Great write up Colin !

[cal844]

Could Admin sticky this??

[colinu]

Thanks Doug... TMF is all about sharing :)