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MG MGB Technical - Speedometer Error

I can't find an answer to this one in the archives. I am hoping there is an expert out there who can point me in a right direction. My speedometer reads 15 mph high. This was confirmed using my wifes new car as a pace car on the interstate at 60 and then 70 mph. At lower speeds (40mph) it reads about 10 mph to high. Using the mile markers as a measure also confirms I have a problem.

The car is a 73 MGB with a LH overdrive (SN22/61972 013309). I have a correct SN-5227/12 1280 speedometer. Tires are 165R14. The speedometer has no flutter and operates smoothly over all ranges. I performed the standard test. Carefully marked off 52.8 feet and came up with 12.75 x 100 = 1275 (close enough). This all tells me that when I am pushing the car through the 52.8 feet everything looks good but at speed the unit reads high.

Based on all of this the speedo should be balls on accurate!

I am guessing this needs to go to a repair shop. Any recommendations (USA)? I'm afraid I will get the unit back from repair and have the same problem.

Regards
Bob in Atlanta
RES Schultz

Looks like you need a 1280 TPM speedometer versus the 1000 TPM one in your car. Based on the info provided, the numbers work out about right - at 60 mph it should read 76.8 mph, and at 40 mph it should read 51.2 mph.

Wayne
Wayne Pearson

Check the odometer on a measured mile. If the odometer reads correctly, the speedo needs to be calibrated. If not you have the wrong speedo, or the wrong drive gears were installed in the trans by someone. Nossinger has a good rep for repair/rebuilds.
John H

What colour label is on your OD? A 73 in fact all Mk2 chrome bumper cars should have a black label, rubber bumper have a blue label. The difference is that black label are 1280tpm and blue label are 1000tpm and so must have the matching speedo.

According to my information an SN5227/12 *is* a 1280 as you say, but was originally on USA Nov 67 to Jul 71 overdrive cars and Canadian Aug 68 to Jul 71 overdrive cars. The correct speedo for a USA May 72 to Sep 74 overdrive car is in fact an SN5230/08S, also 1280tpm. As long as the tpms are correct it shouldn't make any difference, and I don't know why non-OD and OD cars had different speedos at the tpms were the same.

If you have a blue-label OD you need a 1000tpm speedo, but if you have a black label as you should then there is something else wrong, and in fact a 1000 tpm OD with a 1280 tpm speedo should read *low*, not high. Incorrect calibration would affect speed, but not distance. I'm not sure whether your distance indication is correct or not, in one place you say that mile markers also indicate you have a problem, but in another you say when you pushed the car through 52.8 feet the distance looked good. If the distance indication is correct, and it is only the speed that is high, then it is simply calibration of the speedo that is out. If distance is out by the same amount then something else is wrong, like diff ratio, wheel/tyre rolling diameter, and you could also have 1000 tpm guts behind a 1280 tpm faceplate.
Paul Hunt

Check the archives on this. Even though you may have the correct speedo for the gearbox and tires it could still be off. I have that problem with mine. It reads about 10 mph high at 60mph. I haven't done checking a lower speeds yet. This speedo was in rough shape when I got the car. The bushing inside was frozen and the gears were all gummed up. I cleaned and lubed everything and it's worked for 12 years or so. I just mentally adjust for the offset. It does have a bit of wobble probably because the cable needs replacing.

From what I can read if all else is correct and you're off it could be due to a weak spring or there may be a problem with the bushing assembly. Finally, at least on the older speedos, the needle is set to match a point below zero on the face (I think the peg can be pushed in to do this).

The solutions for a weak spring is an adapter box by a speedo shop or rebuild by someone like Nossinger. You could probably reset the needle if you want to open the case.

I'd love to hear how you come out.
Robert McCoy

Thank you all your your comments

Wayne. I have the correct speedometer (1280)

Paul. As I pointed out I have a LH overdrive, the correct speedo and tire size.
The OD and speedo came out of a 1971 MGB. At highway speed 70 mph it reads 15 mph to high. At 40 mph is reads 10 phh to high. When I push the car 52.8 feet it is spot on.

John, Thank you for your comment. I have the correct drive gear in the OD.

Thank you Robert for your comment. I am leaning toward your idea that I have a weak spring. The error increases at the speed of the car increases. Rebuilding a speedo professionally is a expensive proposition. I want to understand what is wrong before I spend the money. I am about 90% convinced i will get the rebuilt unit unit back with the same problem. I have not found anything in the archives that help me understand what is going on.

Regards to all
BOB
RES Schultz

Bob,
The odometer is a simple gear-driven device, unaffected by the spring in the speedo.
As John says, check the odometer over a measured mile. If the reading is correct, then have the speedo rebuilt. End of story.

I don't know about the US, but when I recently had my Magnette speedo recalibrated due to the same problem the cost was around $100.
Regards,
David
David Overington

BOB,

I have the same set up as you. That is, a 1280 speedometer, and a black-label top-fill overdrive. My tires are 165 x 14 and I think they are a 75 ratio. I don't know the numerical indication of the R, unless you meant SR, which I think are obsolete.

It certainly is possible for your turn per mile check to be correct, while the speed is off. I suspect your odometer is pretty close to right even though the speedo is off.

I haven't fixed mine, but here are the figures. Based on several 100-mile checks against the highway mile posts, my odometer reads about 96% of actual (100 indicated for 104 actual). But, based on a multipoint check with a GPS, the speed reads 133% of actual (f or example, 60 mph indicated at 45 mph actual).

Actually, I did "fix" it with a yellow paint stick that writes just fine on glass. Worked great till it quit entirely. I am guessing that the cable broke, but I haven't looked yet.

Charley
C R Huff

I always thought that the speedo differentiated the rotational speed by spinning a magnet and balancing the torque with a spring, so if the spring weakens then it will overead. If it was just a gear train the needle would rotate all the time at whatever it was geared to.
Stan Best

Stan,
You are correct. Over time the spring weakens, giving a high reading. Easily fixed by a competent instrument fitter.
On the other hand the odometer is driven by a direct gear train.
David Overington

As I said, if distance is correct but speed high (it wasn't clear from your original post) then it is simply speedo calibration. At its simplest this could be rectified - for a band of speeds anyway - by simply tweaking the spring.
Paul Hunt

Had the same problem (exactly) in a 70 MGB with OD. We sent the gauge to APT Instruments. Came back working fine.

Contact APT at www.gaugeguys.com No financial interests, etc.

warmly,
dave
Dave Braun

Thanks to all. Based on the helpful comments I received I now have a handle on what is going on. Using my GPS I took the car for a drive and checked speed and distance. After 37.7 miles (according to my GPS) the odometer on the car reAd 37.2 miles. A modest 1.3% error in the odometer. On the other hand the speedo was well off the mark.

MG 15 mpg GPS 5 MPH
MG 30 GPS 22
MG 40 GPS 27
MG 60 GPS 45
MG 90 GPS 73
MG 95 GPS 76

This confirms I have the correct speedo but the spring has weakened. Time for a rebuild!

Thanks again
Bob in Atlanta
RES Schultz

Try this website for all sorts of info on the speedo. http://www.mgaguru.com/mgtech/dash/st202.htm.

Somewhere on the web I found a 27 page documents called "Repairing Jaeger & Smiths Speedometers" by Anthony Rhodes. Very fact filled. Written around 2002. Try Google.
Robert McCoy

This thread was discussed between 16/06/2008 and 22/06/2008

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