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MG MGB Technical - Speedometer inaccurate.

A couple of years ago I fitted new wheels and tyres, stepped up to 15 inch wheels tyres to 185/65/15. And of course now the speedometer is really useless. I have a meter that reads my speed using the satellites , when my speedo reads 30 the meter reads 33 at higher speeds it is out by about 20%. I’ve heard that the Speedo can be re calculated, does anyone know if and how this is done.
Trevor Harvey

You can send it away to be recalibrated, I know of three places.

Speedy Cables is often mentioned as a source of speedo repairs, but there have been complaints of these taking several weeks.

JDO Instruments offers a 48 hour turn round which has been verified by members of the MGOC MGB Technical forum. But for the purposes of balance someone ringing JDO about a repaired speedo that had failed had the phone slammed down on him with the words "I'm 80 years old and I don't need this", and someone else said he had used Speedy Cables with prompt return.

I used Speedograph Richfield to recalibrate Vee's speedo as part of a 5-speed conversion. It had to go back a second time as they hadn't replaced dirty odo wheels, and when refitted it over-read by a full 10% whereas previously it had only been 1 or 2% which was annoying, until I reset the pointer on the spindle.
paulh4

I believe that Speedograph Richfield have now ceased trading.
Dave O'Neill 2

Ah well one less decision to make :o)
paulh4

I have purchased one of these electronic gadgets that work from satellites that show the speed your doing on the windscreen so now I’m able to keep within the law. Not ideal but it’s doing the job.
Trevor Harvey

The variation is interesting. It isn't the usual linear one that comes from incorrect calibration, or the % difference wouldn't increase. It also doesn't arise from the different wheels and tyres, as 185-65x15 is the same overall diameter as 165x14, and only 3% different from 155x14.

One change that produces a difference of that order (20%) lies in the various speedo ratios between overdrive,non overdrive, early and late gboxes.

Or there is always the possibility that the other instrument has an issue.
Paul Walbran

OD/non-OD only affects the speedo on 3-sync, 4-sync speedo drive ratios are the same for both. Even then it was only about 2% between the different speedos (1020 tpm and 1040). The big difference comes if you have a 4-synch CB speedo (1280tpm) on a CB 3-synch gearbox/OD or on an RB gearbox/OD which should have 1000 tpm.
paulh4

I have used JDO a couple of times with no problem, I think it has been passed on now to someone else. When I had a subsequent problem due to my inept re-fitting he sorted it and posted it back FOC.

Always had very helpful chats with the new owner and very rapid turnround.

Mike Dixon

Paul, the odd one out is the rubber bumper overdrive fitted with the red worm, which has a ratio of 6:20 = 0.3
The workshop manual doesn't note this (at least not my edition which was printed about 1980) so I found it out the hard way when I fitted an od box to a customer's 72 model and found the speedo about 20% out. Surprised, I reconsulted the manual and redid the maths, which said there shouldn't be a significant difference. Further digging revealed the cause: the donor box was from a rubber bumper car, and counting the teeth on the gears in stock revealed the WSM didn't update from the original blue gear ratios. (As we are on km here, all was not lost as the difference was conveniently such that if you doubled mph reading on the speedo then it was within 2% of km/h!)

So the manual, while extremely good, isn't infallible. One other thing I found was the elevations to the subframe bolt holes on the front chassis rails. I had gone end over end in my B in a rallysprint and during the course of putting the mess right found these dims on the LH chassis rail didn't match the manual. Given what had just happened, I naturally assumed there was more work to do there. But I couldn't see any creasing or deformation that would explain it, and the rail measured right everywhere else. Puzzled, I turned to the (new) RH to compare (this was 1982 so it was still a case of such parts being residual original stock) and found it matched the LH rail exactly.
From memory, I found that if the fractional parts (either the whole fraction or just the numerator) of the WSM dimensions were swapped, everything agreed.

I had a similar puzzle with the shape of the LH wing where it met the scuttle, had about 6mm runout vertically. Again I assumed my was-battered B was the problem. Took numerous dimensions under the scuttle from a number of other Bs: all the same. Made a profile of the wing's vertical curve and offered it up to the RH wing and found them different. Again, being 1982, this was residual original stock held by NZ's BL network, so I took it back and swapped it for another. Same problem! So I gave up and bullied it into shape. Every roadster LH wing I have seen since has the same issue, last one we fitted was about 5 years back,so I guess we can say that the Heritage panels are faithfull to the original!
Paul Walbran

Paul - from my researching that 20% inaccuracy came about because an RB OD was fitted to a 72 i.e. a CB. CB both OD and non-OD use a 1280tpm speedo and RB both OD and non-OD use a 1000tpm speedo.

CB do use different gears in OD and non-OD but the ratio is the same at 1:2.6. Similarly RB use different gears in OD and non-OD but again the ratio is the same at 1:3.333.

E&OE!
paulh4

Yes, that's the one. Fitting rubber bumper od box to chrome car or vice versa. Happens surprisingly often when overdrives are retrofitted.
Paul Walbran

This thread was discussed between 24/06/2026 and 08/08/2026

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