Morris Minor Owners Club

Its probabally too late, but I'm having some real problems getting my head round some maths to work out what TPM speedo I should be using.

I'm ditching my current wheels in favour of 15" Jobs (currently sitting with 205/60s) and trying to work out what TPM to use....

First task, check your present speedo against a sat nav to see how far out it is. It may not need changing.

Assuming present speedo is correct (unlikely!) - measure circumference of existing wheel tyre combo, and ditto for new combo. Then take tpm of speedo, multiply by existing size and divide by new size! Simples.

mike.perry wrote:First task, check your present speedo against a sat nav to see how far out it is. It may not need changing.

Its only ever correct when I'm stopped

Doesn't work at the moment....

Assuming present speedo is correct (unlikely!) - measure circumference of existing wheel tyre combo, and ditto for new combo. Then take tpm of speedo, multiply by existing size and divide by new size! Simples.

so

(Ce*TPM)/Cn should give the correct TPM? Whats the correct TPM for a standard 1098 with 4.22 diff? - 1408?

If you get probs with the TPM Speedy Cables, Richfield Speedograph will re calibrate for you if you fill out one of their forms, pretty reasonable on cost too, if the free source of minor ones are unavailable

what tyre size will you be using as I should be able to work it out using the method I used to calculate mine . I have the info written down somewhere just need to dig it out & i'll post it up

See above 205/60/15

Mogwai, I just love your signature photo! very different!

Yes - 1408 or thereby is correct for a 1098 with 4.22 diff - and standard tyres of course. Yours may not be much different - how do the tyre circumferences measure up ?

See above 205/60/15

Balloons then

I reserve the right to be horribly wrong but this is the method I used to calibrate my Speedo
1st you need to calculate the tpm of the tyre
If you are using 205/60 15 should give a circumference of 77.5”(courtesy of tyre bible)
1mile = 63360”
63360 / 77.5 = 817.5 tpm
Next will need the ratio from the Speedo to the axle for a std box & 4.22 diff will be
5 / 13 x 38 / 9 = 1.623
The tpm should be
817.5 x 1.623 = 1326 tpm

my brain hurts now

According to http://www.miata.net/garage/tirecalc.html


175/65/13 (current fitted) is 69" 917 Reves per mile
Standard is 72.7" - 872 revs per mile
205/60/15 is 77.6" - 817 revs per mile

So using my interpretation of your maths BMC - (Ce*TPM)/Cn

(72.7 x 1408) / 77.6 = 1319 which is quite a significant change - does anyone know if theres a similar rated speedo?

(well, its 1319.0927835051546391752577319588 but thats prob taking it a bit extreme!)

6.7% difference in turns per mile so I would guess a stock 1098 speedo should be fairly accurate

alanworland wrote:Mogwai, I just love your signature photo! very different!

cheers. took ages to get it right as its 4 pictures stitched together turned out quite well though

Matt wrote:6.7% difference in turns per mile so I would guess a stock 1098 speedo should be fairly accurate

Yes, but it says that the 13" ones I have is a 5.1% difference, but when the speedo was working was hugely out

Judge the speedo accuracy from the mileometer readings. The actual speed reading can be all over the place - especially if the needle has ever been off the spindle! If the mileometer is good within a small % - then the speed 'should' be correct - may have to twiddle the needle to get it right!

The standard 1098 speedo was well out on my Minor with 4.22 diff and 155x14 tyres. It is spot on with 3.9 diff and 175x13 tyres. Therefore a TPM calculation may not give you the best answer as it does not allow for speedo error, hence the recommendation to check with a sat nav.

Yet my 1508 speedo was almost perfect with 4.55 diff and 155 x 14 tyres. The mileometer was correct to 0.1 miles in 10 !

Yet my 1508 speedo was almost perfect with 4.55 diff and 155 x 14 tyres.

Although you thought you had a 4.22 before you changed it

Indeed I did! But the speedo matched the ratio - which is why I had no reason to doubt anything ws wrong. The 4.55 ratio did help to explain the nippy perfprmance - and poor MPG!