Any Maths Boffins Out There?

[v8s4me]

I'm trying to find an alternative speedo transducer for my V8S. The OE unit has 20 teeth and the diff is 3.36:1

I can find these alternatives:

22 teeth for 3.09:1

22 teeth for 3.9:1

So what is the "net" effect at the speedo of running one of these compared to the OE item?

Does anyone know definitively whether the V8S had a 4 phase or 8 phase transducer?

Thanks in advance.

[FarmyardPants]

The speedo toothed wheel rotates with the drive shaft/road wheel, the final drive ratio is irrelevant (just affects the revs for a given road speed and gear). All that should matter is the number of teeth. If you use 20 teeth instead of 22, your speedo will under-read by about 10%.

Edit: just noticed you're looking at using 22 teeth instead of 20, so the speedo would over-read by 10%.

Edited by FarmyardPants on Thursday 9th July 21:24

[v8s4me]

Very helpful; thank you. So now I just need to know whether it is 4 or 8 pulse. Presumably if the speedo head is calibrated for 4 pulse and I fit an 8 pulse it will read double, and vice versa?

[phillpot]

FarmyardPants said:

The speedo toothed wheel rotates with the drive shaft/road wheel

but not on a V8S, it is driven from the back of the gearbox, final drive ratio will make a big difference!



Won't 22 teeth under read compared to 20? more teeth will take longer to complete a cycle = slower?

Edited by phillpot on Thursday 9th July 23:27

[FarmyardPants]

phillpot said:

but not on a V8S, it is driven from the back of the gearbox, final drive ratio will make a big difference!

Ah. In that case forget everything I said! Doh.

phillpot said:

Won't 22 teeth under read compared to 20? more teeth will take longer to complete a cycle = slower?

Edited by phillpot on Thursday 9th July 23:27

I think it will over-read because the speedo will see more pulses. Eg for one revolution it will see 22, which it will think is one revolution (20) plus 2 pulses from the start of the next one, ie it thinks the car has traveled further in the same timespan.

If the toothed wheel rotates with the prop shaft, I think the calculation is

Overread = (new teeth/original teeth) * (original ratio/new ratio)

Since using a higher final drive ratio will make the car run slower for a given prop shaft speed.

[v8s4me]

FarmyardPants said:

Over-read = (new teeth/original teeth) * (original ratio/new ratio)..

(22/20)x(3.36/3.9) = 0.94

So does that mean the greater number of teeth on the transducer gear is cancelled out by the (actual) lower diff ratio?

My brain hurts...I'm going to the pub

[FarmyardPants]

Yes . The under-read caused by higher diff ratio is more than the over-read caused by the extra teeth, so 6% under-read overall. Nice weather for the pub .

[v8s4me]

Thanks. Just need to find out what the pulse rate is now.

[phillpot]

Bring back speedo cables