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MGA MGA Speedo Calibration

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Ray7Smith

Senior Member
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Just put a new set of minilites and 185/70/15 on my "A". The speedo is reading high. Is there a formula for subtracting from the reading to compensate for the larger tires?

Ray
 
compare the circumfrence of the original tire to the new tire, both inflated to the correct pressures. convert the difference of the two circumfrences to a percentage of the original, larger tire and subtract that percentage from your speedo readings. circumfrence is directly proportional to revolutions per mile so a tire that is x% smaller will make the speedo read x% faster.
 
Or... use an online mapping service to plot a course from your home to a spot several miles away. Record your odometer/trip readings before and after you drive to the distant spot and back. Compare the total miles the online service predicted to what your odometer really shows. Dividing one of these values by the other will give you a ratio that you can apply. This will tell you how your tires in their current condition affect the odometer accuracy. If your speedometer was relatively accurate (when compared to the odometer) before, you can apply the odometer ratio you determined above to the speedometer reading as well.
 
Get a gps and drive 60mph by the gps and see what the speedo reads, Then take the face off the speedo manually push the needle up to where it was reading at 60mph on the gps. Hold the stem behind the needle with a small pair of neeedle nose pliars and use a fork and remove the needle and put it back on to whre the needle is reading 60mph. Then take it for a test run with the face still off and compare the gps to the speedo and see how close it is. You don't have to use 60mph but that is what I used. I had changed tires, diff ratio, and added a 5 speed so mine was way off.
 
Or you could just have the gauge calibrated to your new tire size. Then your speed and distance will be even more accurate than before, and the needle will be rock steady.
 
Steve's and Kim's methods both work. Kim's method will however not give you an accurate odometer if that's important to you and outside of the "calibrated" area on the gauge you will have errors to mentally compensate for. If you do send the gauge off for re-calibration to match the new tire size, the shop (whoever you choose) will want you to provide them the kind of distance measurements I was talking about. To calibrate the odometer accurately requires internal gear changes so accurate on-road measurements are required for the best results.

An alternative to changing the internal gearing of the speedometer is fitting a ratio box inline with the speedometer cable. The distance measurements are still required and the speedo shop may still want the instrument for calibration, but no internal gear changes are required to change the odometer performance.

Internal recalibration would be the most expensive but cleanest. The ratio box would be a little less expensive but you've got the box to mount somewhere. Home recalibration of the speedo is viable but has errors the further you move away from the speed you choose to calibrate at. What you choose all depends on your end goal and budget.

Of course... you could use a Sigma bicycle computer as a second speedometer. I have one in the Mini and I think Kim may have installed one in his Spridget.
 
So... how do you think they (Palo Alto) came up with the distance of 52'-9.5"? When I multiply that out in millimeters I see a MPH to KPH conversion factor that appears to be off by a factor of 10.
 
It's 1/100 of a mile

They use it to calibrate your spedo.

I had to do the same thing when I switched to 15" wheels.
 
I second Palo Alto Speedometer. They do a lot more than speedometers. Sent them my clock & some other things from my 1973 BMW, always good work & reasonably prompt.

Colin
 
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