speedo pinions?
Turbo Mini Forums

hi, iv just put back together a 1275gt a series gearbox after changing a few parts. i will be using this in my mpi.

there wasnt a speedo drive (the part that goes into the end of the speedo housing on the end of the gearbox, which is attatched to the speedo cable, and secured by the half moon type clip.

am i able to use the drive from my mpi box, are all of these interchangerble?

many thanks

Interchangeable.... technically no !!!

There are different coloured nylon wheels (the one that connects to the cable) depending on the number of teeth on the steel "worm" and the ratio you want between them.

You need to start by working out the ratio you want, based on speedo TPM and FD ratio, and see what gears are available in a "matched" set.

In practice though, I have used mis-matched sets and got away with it, the most important bit is to get the ratio right so the speedo reads right.

Schrödinger's cat - so which one am I ???

as rod says you can get away with mis matching them...

but IIRC the MPI ones are different (vastly diferent helix angle) and cant be mixed with the others if you have a look on mini spares there is one that the part no srts TMX or somthing similar, thats the odd one.

ok, cheers. im not sure how to work this out.

i changed the final drive from 3.4 to 3.2.

the drive i have at the moment is for the 3.4

i am running 13 inch wheels. the speedo itself is standard sportspack.

can anyone advise on what speedo i should be looking t get to make the speedo read acurate or 'near enough!'

cheers!

Read this:

http://www.guess-works.com/Gearbox/Technical/speedo.htm

explained in detail

hmmm,

speedos re a fooking pain in my experience, i spent ages finding all the right bits to get it so read right, by the calcs mine should read 2mph fast at 140...

its nearly 10mph fast at 100.

I'd suggest trying what you have against a GPS then going from there, unless you know it will be masively out.

i need my hand held on this one.

not got a clue! the worm i have fitted is for a 3.4 final drive ( + close ratio gearset).

the only pinions i have spare are mpi, so they wont fit. just need to know what pinion i need to fit to give the speedo reading a near enough reading.
i will have to obtain/ buy this anyway

wouldnt mind 10mph fast at 100, mannly cancerned about it showing me 30 when actually doing 40

near enough will do, as long as its not massively out.

ok, fist thing,

have a look on your speedo face for some small digets, 1000, 1280 or somthing like that, its your turns per mile. then we can estimate it.

wil pop out now and see. are they present on the mpi dials?cant say iv noticed them before

What you really need to know is the number of teeth on your steel gear (the one inside the box).

They are 5, 6 or 7 teeth (unless anyone knows of any more...)

These really determine the best helix angle of the plastic gear, and also allow the ratio to be calculated.

Schrödinger's cat - so which one am I ???

there is nothing on the face of that sort. just checked. they are standard mpi sportspack dials. i have a few spare ones down the garage, could the case be marked or something? cheers

rod i can pop down the garage and count. will be around 20 mins i expect

the spiral gear, seems to have 4 on it?

the spiral gear, seems to have 4 on it?

I've never seen a 4....

Metro sourced boxes usually had 5

Early Minis 6

Late Minis (with the higher FD) apparently had 7 although I've never seen one personally.

Re-check that 4.....

Schrödinger's cat - so which one am I ???

4 didnt seem right to me. just recounted, its definately 6.

OK,

Use Guesswork's calculator with a 6 - I haven't looked at it recently but I thought it gave the TPMs of the common speedos even if the number isn't marked on it.

Schrödinger's cat - so which one am I ???

cheers. never even looked at the pinion/speedo part of that. only ever had need to calculate /compare actual ratios. looks like im after a 6/16. will post in the wanted ads for a couploe of days then order one. cheers

just looked on mini spares website. their 16 tooth pinion (green) states 'was used on 3.1 final drives with DAM6028 drive spindle which had 7 teeth. '

would this be usable with the 6 tooth worm that i have?thanks

I have used all speedo pinions with all speedo spindles. There is only one exception, and that relates to the 7 tooth spindle. There are infact two 7 tooth spindles. One has a smaller outer diameter than the other. This is excluding all Metro gears as some of those have a larger square drive on the pinion shaft.

I had a nightmare with the speedo I have. I manualy calculated the ratio from the turns per mile, final drive ratio and wheel diameter. This was then checked against the link above to be correct, yet the speedo read miles out. I had even taken into consideration the rolling radius of the wheel.

What I ended up doing is fitting the varius gears spindle combinations in order of ratio, to narrow in on the final choice, the very last one, no other ratio available, and if memory serves its an 18 tooth pinion from an MPi and a 7 tooth spindle (lol can't really remember) from an older box.

The speedo now reads dead on correct with only 1 or 2mph error at 90mph. This makes me think the speedo has been calibrated in the past as I have never seen a Mini speedo read so accurately.

those problems above are why it is preferable to calibrate the speedo with the drive gears and not the speedo head itself!

My dad did some work a Long, Long; Long: Long time ago for John McGavigan in Glasgow. Aparently speedos are calibrated to be accurate at 30mph - 10% + 0%. In this day and age, I have noticed that most cars speedos are calibrated at -10% at 30mph and I think its a conspiracy

Edited by Sprocket on 29th Sep, 2009.

ideally i would need a 7 / 15

is this a standard setup? anyone know what was fitted to the mpi minis?

sorted. i believe that 7/15 was fitted to the mpi. have a few of these units, will strip one out over the weekend. will look to use the 6 worm and 15 pinion.

cheers for help

Edited by welshdan on 29th Sep, 2009.

Nearly Sprocket - Speedo calibration is within -0% +10% ie if 30mph is your calibration machines output signal/drive (usually is, and is usually the first of 3 speeds used) the speedo should read between 33mph and 30mph. As you say they are usually nearer the 33mph mark at a true 30mph - so when driving at an indicated 30mph you are actually doing 27.27mph.

Obviously when you get round to 70mph indicated this means you are actually only doing 63.6mph!

You can normally see marks on the very edge of the speedo (in fact most gauges) that show the limits of calibration at each of the tested values.