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MG MGB Technical - Calibrating a Speedometer

The speedometer in my 72B (w/OD)when purchased did not work. The needle was broken and upon opening it up I discovered 2 plastic gears were essentially toothless.

Original speedo numbers: SN5231/04, 1280, w/401 on the back, 120 MPH.

I purchased a used one and installed it, without paying too much (o.k. - none) attention to the wide variety of speedometers out there. It worked, but only up to around 40-50 mph, no matter how much faster I went it didn't increase. I opened it up and followed a printout on speedometers and tried to clean it. No disassembly, just canned air and little brush. There was grease from cable inside and dirt. After all my "expertise" attention, it worked the same as before and now the trip meter didn't work.

Second speedo numbers: SN5226/09, 1376, w/118 on the back, 100 MPH.

So, again (and without adequate investigation) I purchased a used speedometer that looks nice and clean, increases and decreases through all speeds, the trip meter and odometer work fine - to a point. I think speedometer reads about 65mph when I'm doing 50 which is obviously also showing too many miles for actual distance driven. I think the speed difference is about the same at 30mph, with speedo showing 45mph. These are approximate, but I am usign a traffic speed indicator a few blocks from my house to test with, so I'm close.

Third spedo numbers: SN5230/13, 1000, w/245/20 on the back, 120 MPH.

I haven't dared open it up, based on my last learning experience, and I haven't consulted the speedometer repair printout I found on MG Enthusiast, as this may give me unfounded confidence!

So, the question is; 1) am I even close to the right numbers for a speedometer that my 72B w/OD should have?, 2) is the high reading something that can be easily re-calibrated?, 3) or will part replacement be necessary?

Recommendations please. Thanks.
R.W Anderson

Okay, I now have eye strain from going though the archieves.

If I understand things correctly, my 72B w/OD should have a 1280 speedometer, not the 1000 I just installed. And I do have about a 28% over stated speed error (going 50, showing 65). It sounds like changing the speedometer parts around is a major job and we know how well cleaning went for me!

Past posts (back around '98-2000) suggested buying a new 1280, or going to a shop to have a adapter gear make for $100 or more to attach to the cable.

It also sounded like to get a 1000 speedometer to work in a 1280 vehicle I need to change things in the speedometer and the OD unit too!

So unless someone has a simplier new idea, it appears I need to keep my eye out for an operating 1280, or I just tape a sign to my speedo that shows 4th gear and OD tach equivalents (18 mph per 1000 rpms).

Oh, and to better clear which transmission I have, as this seemed to be an issue in the old postings; I have a dip stick to check oil level in trans that is behind my radio counsil, and this is the fill point too.

Looks like I'm back to ebay.

R.W Anderson


You have a couple of different options close to home. You can first acertain which OD gearbox you have (the label on the side is black or blue, I could check which is right for your car); but even if you do that your tire size may provide a different tpm. You can sell that horrible old OD gearbox to me and I'll trade you with a normal one (ha-ha); or you can simply have one of the Speedometers you have recalibrated. I like option three best because it will smooth out your instrument and get it reading accurately. A new angle drive adapter won't fix a badly acting needle.

Usually, the odometer portion works properly regardless of the waving or stuck needle syndromes. You can drive a known stretch of road (I-35W close to us come to mind, it has accurate tenth mile markers from just north of the twin cities to the crosstown split) and see how far you are off. 10 miles will be enough to know. Or, you can put a flag on your speedometer cable and push the car 52.8 feet. With your set up you should see 12.8 turns in that distance, less if you have larger tires. Take that information, regardless of how you determined it to APT instruments in Bloomington for about $225 they will calibrate your gauge or repair the one you messed up. Tell 'em I sent you! They do extremely good work. No interest, etc.

good luck,
dave (who met you at Osceola last year)
Dave Braun

If you have a top fill gearbox, then you should have a 1280 TPM speedometer. It doesn't matter if the gearbox is O/D or not. Same speedo drive ratio for O/D and direct drive. I can't remember the numbers, but maybe the speedo drive is 5/21 (?) for the top fill units. I can't remember at all the speedo drive ratio for the side fill boxes at all, but the speedometers are 1000 tpm. You can't easily change the speedo drive ratio in either box, the pinions are matched to the output shaft gears. Your best bet is to get a 1280 tpm. I have several 1280 speedos and I could use a 1000. I'l send you an email about that.
Steve Mc


Well as you can see from my postings I have a perfectly functioning 1000, no bounce and trip meter resets smoothly. I'll await your email and we can see what we can work out.
R.W Anderson paste and copy this to your address bar. he rebuilt and a reclibrated an mgb speedo for my midget for

When I converted my '67 to the 4 synchro o/d unit from a '72, I used a speedo that was told to be correct by my distributor. It was a NOS SN6144/00. The speedometer and odometer are dead accurate. Ray

This thread was discussed between 20/04/2007 and 23/04/2007

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