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MG Midget and Sprite Technical - Speedo recalibration formula.

Some time ago I bought a recalibrated speedo from MGBhive as mine had ceased to work and I was running 175/80x13 tyres so needed it calibrating to suit the tyres.
I have now fitted a Type 9 gearbox and 175/70x13 tyres, so it needs doing again. Trouble is I've forgotten what the procedure is regarding measuring the distance travelled for one revolution of the wheel etc.
I have spoken to MGBhive and also Speedycables and have been given different information from each source. So. I was wondering, has anyone here got the definitive answer? Thanks, as always.

Bernie.
b higginson

I'd use the speedy cables method. It worked well for me.

The other method of jacking up a wheel is less accurate.

I took this direct from the speed cables website.

---- you need to calculate the required revolutions per mile.


(1) Take the measurement from the centre of the hub of a drive wheel to the ground, with the tyre pumped to normal pressure.

ANSWER:(1)__________ distance in Inches

(2) 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

Answer:(2) __________ distance in Inches

(3) Now disconnect the speedometer and place a cardboard arrow on the end of the protruding inner speedometer cable.

Put a chalk mark at the bottom of the measured wheel and then push the car straight forward (with gear in neutral), counting exactly 6 revolutions of the wheel, whilst a partner counts the number of times that the arrow on the cable revolves.

ANSWER:(3)__________ number of turns of the cable. (N.B. include parts of a turn e.g. seven and a quarter turns)
Lawrence Slater

Lawrence. Thanks for that. I assume that one then gives all the answers to the firm doing the recalibration.

bernie.
b higginson

Hi
Yes these are the answers they want. but no harm calcualtin it youtself just for interesting and checking that the number of speedo turns is correct.
Lawrence Slater

My tyre chart lists 175/80 has 610mm diameter, 175/60 as 575mm - a difference of 5.7%. It would be a useful cross-reference on you calculation. But then again the original speedo might have been out.
Paul Walbran

This thread was discussed between 03/08/2011 and 04/08/2011

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