Speedo logic. TPI and speedo heads

[cardiffrob]

I'm most of the way through building an Austin 7 (1930's) but still need a speedo head.

The gearbox drive is set up for 1080 TPI speedo dials but they aren't quite right for what I need, being tiny on the original car. I can find lots of 1000 TPI speedos and lots of circa 1600 TPI ones but can't quite work out in my noggin what would happen if I bought a 1600 dial and ran it on the original drive system. Would it under-read or over-read?

Does anyone know for certain which way round it goes?

What about KPH ones which, presumably, run at 5-8 or 8-5 times as fast? The ratio of MPH to KPH is about 5-8 or 1000-1600, which might help me out.

Brain fade....

 

[rtbcomp]

If the gearbox puts out 1080 turns/mile and your speedo only needs 1000 turns to record a mile, then it will read too fast by a factor of 1080/1000.

So Indicated Speed = Actual Speed x 1.08, therefore Actual Speed = Indicated Speed/1.08

On a Kph speedo the mph is 5/8 of the actual reading, eg a reading of 80 kph is actually 50 mph. This will apply if you have speedo that needs 1080 rev/km

If you have a mismatched kph speedo, say 1000 rev/km you'll have to apply both corrections, so the actual speed a 1000 km/rev speedo will show will be Indicated Speed/1.08 as above, in kmh, you will then have to multiply this by 5/8 to get the actual speed in mph.

I think.

 

[Robotstar5]

Is it worth getting it re-calibrated to match your drive? - link

 

[cardiffrob]

Thanks, guys.

Doing it all on a budget (aren't we all?) so I'll have a look at 1000tpm ones on Ebay again. 1600 ones would adjust the ratio even further the wrong way if I put one of those MPH/KPH adaptors on it, I think.

 

[The Devils Reject]

What size are the dials in an Austin 7? Anything British made from that era and up to mid 80's is likely to fit.

 

[cardiffrob]

Just over 2 inches for the original but I can take anything up to the 4 or 5 inch Mini/Minor ones. I found some that looked ideal until I realised they were 24v electric ones. Not sure if I'm going to stay 6v or convert to 12v. Currently no dashboard fitted.

 

[The Devils Reject]

Series Land Rover ones are about 4 inches across. A LWB one is 1408tpm (from memory)

 

[The Devils Reject]

Just over 2 inches for the original but I can take anything up to the 4 or 5 inch Mini/Minor ones. I found some that looked ideal until I realised they were 24v electric ones. Not sure if I'm going to stay 6v or convert to 12v.

Shouldn't make any difference what voltage they are. It's only the bulbs that make the difference as the speedo is mechanical. Unless they are powered by electric...

 

[Cobbler]

From memory series 2 Minor ones were 1000 TPI. Could well be wrong though, last had a Minor about 40 something years ago. This was the later version with the BMC dash, not the Nuffield one. 1000 ones were different TPI if I recall. As most speedos over read by about 10% this might not be too far out.