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MG MGA - speedo recalibration

My speedo generally reads up to 10mph too slow which means that I have to really concentrate when I am in a speed limit area.

The error probably stems from the 5 speed gearbox conversion the car had before I got it.

I have contacted a couple of companies who specialise in instrument recalibration and both have recommended that I disconnect the speedo drive from behind the panel and fix a dial to it so that I can record out how many times it rotates in relation to a measured number of wheel rotations.

One firm told me to jack up one rear wheel, rotate the wheel exactly 20 times and count the resulting rotations of the speedo drive to the nearest 1/8th turns.
Another advised against this method as they say it gives a completely false result. They recommended that I measure the rotations of the speedo cable caused by pushing the car along the road until the wheels had rotated exactly 6 times.

Has anyone carried out either of these process and if so, how did it turn out?
Cheers

Colyn

Has anyone done

I understand the principles behind this but I wondered if anyone has actually done it
Colyn Firth

Colyn

I did the former measurements, as detailed by Speedy Cables. They then did the recalibration. It was spot on when I subsequently calibrated it against my Sat Nav.

When you say 'fix a dial to it' I presume you mean a pointer so that you can count the cable revolutions when you move the car a set distance over the ground.

Steve
Steve Gyles

The first method described was from Speedy cables as you thought Steve, it does seem a little easier to carry out than the 2nd method.

I will give it a try and let Speedy have my speedo to fix up, probably I will do the measurement and then wait until next winter to have it recalibrated.
Cheers Steve
Colyn
Colyn Firth

Disconnect the speedo cable from back of instrument, and attach a piece of tape to the inner cable. Make two marks on the pavement 52-feet 9-5/8-inches apart (1/100 mile). Push car from one mark to the next, counting turns of the cable. Multiply by 100 to get turns per mile.

For MGA with original 4.3 final drive this should be 1450 0r 1440. That number is found on the face of the instrument below the main odometer window. If you have different number of turns per mile you can have the speedometer rebuilt with different gearing for the odometer drives (and speed reading adjusted as well), or install an intermediate speed change gearbox somewhere long the drive cable.
Barney Gaylord

Colyn,
This is interesting because i used Speedy Cables who gave me the six rotations method. This factors in the effect of the true outer circumference of the tyres and the distance thus travelled. Any method that does not allow for this is flawed (different sidewall ratio etc will affect circumference and thus, distance) My speedo is spot on according to both a Sat Nav and two modern cars running alongside.
regards
Colin
C Manley

Speedy Cables formula for computing Turns per mile is:

(cable turns in 6 wheel turns) X 1680 divided by the radius of the tire measured from the hub center to the ground.

TPM =(T X 1680)/ R

The formula worked for me on my 3.9/5 speed conversion and found a MGB speedo with the right tpm.

Jim
Jim Ferguson

Yes, the Speedy calculation works perfectly.After the 5-speed conversion, I even sent my guage to Speedy in the UK, and had the dial converted to KPH at the same time.
Gary Lock

Im really glad I asked you all about this, some very encouraging responses thanks guys.

When I get the speedo cable pointer set up I will also try Barney's method for calculating the turns per mile. It would be good just to confirm the gearing is what I think it is and probably also help Speedy with their calculations.
The only problem is that I will have to do this on some private car park or something, where I live Barney, if anyone saw the car being pushed up the road they would think it was being stolen!
Thanks all
Colyn
Colyn Firth

Remember that any calibration is as good as the figures you give .
The speedo cable '' pointer '' can be achieved by wrapping a small piece of masking tape around the cable starting with the point in the 12o'clock position.
Also tyres must be correctly inflated otherwise the rolling circumference will differ.
Ron Edmond

I used the method Barney describes after installing my 5 speed. Counted 10.5 revolutions of the pointer over the measured distance. Had my speedo recalibrated to 1050 TPM and it is now spot on per my GPS.
Andy Bounsall

Colyn,
I have been through this quite recently as my speedo was much worse than yours. In the end I used a very small company in W Yorks called JDO-Instruments who I was very happy with. The price was good and he turned it around in just over a week. Its run by a guy called John Ostick who was very helpful. As the car was running, he just got me to drive an aprox 10 mile measured distance (measuring in daily car first) and advise its percentage accuracy - much easier. I didnt need to mess about with the other tests.
His website is a bit complicated but have a look - as you are both in Yorkshire, maybe he is near enough to visit?
www.jdo1.com. Needless to say I am not on commission!!

Good luck
Graham M V

This thread was discussed between 04/05/2010 and 06/05/2010

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