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MG Midget and Sprite Technical - Working out turns per mile

My speedo reads out due to final drive and T9 box change .
Have a few speedos with different turns per mile on the dial. Need to work out what the turns per mile are on my car.

Speedy cables say

Measure centre of wheel to ground in inches.
Then push vehicle one turn of wheel and measure distance on the ground
Then fix cardboard arrow on speedo cable drive and push vehicle 6 turns of back wheel and count turns on the speedo drive.

But then send them the info and speedo for recalibration , they do not say how this calculation will give you turns per mile as indicated on speedo so I can check against speedos that I stock.
Any thought on how to work this out?
R Mcknight

Simple...

If you drive it fast enough with a heavy foot, you can out run any cop radio

Prop
Prop and the Blackhole Midget

There is a John Twist of University Motors video on You tube explaining exactly how to do this.
Chris Hasluck

Why not trial a speedo (not a big job to connect up) and test it against a mobile phone app or TomTom which uses gps. Despite all the calculations, it will come down to that anyway.

There is a very good chap in Redditch who will recalibrate it for you if it needs it.
Graeme Williams

Good one graeme

I like it, alot easier then writing the math out,

I wonder how I can fit that into the dash.. :-)

Prop
Prop and the Blackhole Midget

Graeme
<< There is a very good chap in Redditch who will recalibrate it for you >>
I guess that's Richard at C and V instruments..07747 357 044
Excellent service..just down the road from me.
Took my tacho to him for recalibration in the morning and could have had it back the same day. Fraction of the priuce of speedo cables too
Top service; top chap.
David Cox

All the speedos you have, should have the number written on them. If the numbers are all the same, it won't matter which speedo you use. They'll either all be wrong, or all be right.

The number of turns on the Speedo cable, gives you the overall ratio when turning the wheel. But you also have to measure the circumfrence of YOUR wheel, to see how far that takes you, and work out how many you need for a mile.

That's why they ask you to --- "Take the measurement from the centre of the hub of a drive wheel to the ground, with the tyrepumped to normal pressure"

And, -- "Put a chalk mark at the bottom of the measured wheel also marking on the ground where it meets. Push the vehicle forward one revolution of the chalk mark and record the distance travelled"

As I understand it, the number written on the speedo( from the factory), is the number of turns that spindle makes, for a standard sized wheel and tyre, from whatever car that speedo came from, to make the milometer read 1 mile.

So if you've altered anytyhing, g/box or wheel/tyres, or the speedo is from an MGB or something other than a Spridget, the numbers on the speedos will be wrong.
Lawrence Slater

David: It's Richard Jenkins. From my experience a very genuine bloke offering a good service, fair price and great to deal with.
Having been ripped off by another individual who has been mentioned here from time to time, one can really appreciate the good people.
Graeme Williams

Graeme..yep...same chap. Top job and excellent service. He won't rip you off but just do an excellent job. Recommended without hesitation..
dave
David Cox

Lawerance

Thanks for that info...very informitive

P
Prop and the Blackhole Midget

R McKnight,

To work out the speedo tpm you will need to know:

Tyre size
Rear axle ratio
Speedo drive ratio in the T9 gearbox

If you know these numbers the calculation for speedo tpm is reasonably straightforward - let me know the numbers and I will show you or do it for you!

Richard
Richard Wale

Making a few assumptions - 145/80x13 tyres and a common T9 speedo drive ratio of 22:8 (2.75:1):

4.222 diff = 1399 tpm
3.909 diff = 1295 tpm
3.727 diff = 1235 tpm

Richard
Richard Wale

This thread was discussed between 17/08/2015 and 19/08/2015

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