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
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 ?
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
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.
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!
