Speedo drive help

[spoon.450]

My mk1 has been fitted for years with a Jaeger 115 mph speedo with trip, believed to be from a Triumph, which would like to keep. When fitted with 12” wheels and running a 2.9 diff and 17 tooth pinion with 6 tooth shaft it read reasonably accurate. I am in the process of rebuilding the gearbox and fitting a 3.2 diff and will run 10” wheels. Based on this information can anyone please help me with the calculations to determine which tooth number of pinion / shaft combination would be nearest. I realise this is an unusual ask, but cannot work it out this way on the Guess works site. Thanks in advance, Dave

[jayare]

Does the speedo have a tpm (turns per mile) number printed on the face? Something like 1248 or 1408?

With that info you should be able to select either an exact match or the closest to it on the guessworks calculator.

JR

[spoon.450]

That’s good advice...thanks...I think it has.... will have a look. It could have been played about with inside though ....

[jayare]

Possibly - if you plug in the current diff, tyres, tpm & drive gears on the calculator you'll be able to see if those numbers come up with speed readings that are close to those you're seeing with that set-up. If the calculator says it would read miles out then you'll have a fairly good idea that it has been adjusted in the past. If they're fairly close to what you were seeing, you can then put in the new diff & tyre size and it will recommend the most suitable drive gears.

JR

[spoon.450]

On the speedo face is SN6401/84 and 1184. Does this tell me anything that may help regarding the TPM ?
Cheers

[Thorp]

1184 turns per mile.

[spoon.450]

Thanks for the confirmation. I should be able to work it out on the Guess works calculator now. Thanks for all your replies.
Dave

[spoon.450]

Sorted....5 / 17 required, which I have found in my box of bits
Thanks Dave