Speedo Cable Revs

[NBS]

I have a 33 synchro gearbox. The speedo cable only rotates 5.25 times per 20 revolutions of the road wheel. It seems a bit slow? or not?

[Duncan Grimmond]

What speed does the clock show as you are rolling? Are you able to check indicated speed against a GPS? Are you doing this in a drivable car or "on the bench"

[NBS]

It shows 1/3 approx of the road speed and is on running car.

[Stuart Giles]

Bit difficult to be sure when the final drive ratio is unknown -should be 5.25:1 in there but could be something else.

However, it's easy to check the gearbox drive ratio. In a four speed gearbox it was 4:1 from the factory; the three speed boxes had a higher ratio speedo drive. Should be simple to check that four turns of the prop equal 1 turn of the cable.

Was the '20 revolutions of the road wheel' carried out with one wheel jacked up, or by rolling along the car along the road? If with one wheel jacked up, it would turn the prop twice as many times due to the effect of the diff.

[Stumpi]

Have a look at Speedy cakes website section on calibrating speedos, they did one for me using a milk float speedo and is currently fitted to a successful rallying seven,  
Andy

[NBS]

Bit difficult to be sure when the final drive ratio is unknown -should be 5.25:1 in there but could be something else.

However, it's easy to check the gearbox drive ratio. In a four speed gearbox it was 4:1 from the factory; the three speed boxes had a higher ratio speedo drive. Should be simple to check that four turns of the prop equal 1 turn of the cable.

Was the '20 revolutions of the road wheel' carried out with one wheel jacked up, or by rolling along the car along the road? If with one wheel jacked up, it would turn the prop twice as many times due to the effect of the diff.

Yes 5.25:1. Ah - yes one wheel jacked up.

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Have a look at Speedy cakes website section on calibrating speedos, they did one for me using a milk float speedo and is currently fitted to a successful rallying seven,  
Andy

Thanks for the suggestion. Will take a look.

[Ivor Hawkins]

I think Stumpi meant Speedy Cables!