23 replies to this topic

[alpder]

My speedo currently over-reads 16%* compared to GPS, across the range.

 

A quick calc tells me I can change the existing 16T pinion for an 18T, leave the spindle unchanged, and have a 3% over-read instead. Which is pretty much what most modern cars over-read by, too.

 

I've sourced a NOS 18T/red.

 

Does anyone know how (if) I can fit it without, as for most jobs under the bonnet, taking ludicrous amounts of the car apart?

 

*In fact, the speedo should already be on just a 1.7% over-read: 145/80R10, 3.1FD, 7/16 speedo ratio gives me 1432 turns per mile. Speedo face claims 1408. Guess it's just the age of it. Odd, though, that the return-spring has faded more than the magnets have. Or perhaps BMC were happy to exaggerate the Mini's performance in 1968.

[sonscar]

Fit it through the wheel arch.Some parts will need removing according to your tools and hand sizes.I did it and it did not make me laugh.Steve..

[weef]

As an alternative, there are companies that can repair/ recalibrate mechanical speedo heads. Maybe worth an ask if they can do the work you require, taking out the speedo head will be easier than changing the drive pinion.

[alpder]

Fit it through the wheel arch.Some parts will need removing according to your tools and hand sizes.I did it and it did not make me laugh.Steve..

I've tried before to remove the (pot-joint) driveshaft (to get access to the grease nipple on the direct/magic-wand gearbox linkages) but despite being new in a newly recon gearbox, there's no way they're coming out even with the correct tool.

 

If I take the shaft out of the pot-joint - which is much easier to do - will that help me at all?

[cal844]

Fit it through the wheel arch.Some parts will need removing according to your tools and hand sizes.I did it and it did not make me laugh.Steve..

I've tried before to remove the (pot-joint) driveshaft (to get access to the grease nipple on the direct/magic-wand gearbox linkages) but despite being new in a newly recon gearbox, there's no way they're coming out even with the correct tool.

If I take the shaft out of the pot-joint - which is much easier to do - will that help me at all?

Splitting the pot joint may give more access

[alpder]

As an alternative, there are companies that can repair/ recalibrate mechanical speedo heads. Maybe worth an ask if they can do the work you require, taking out the speedo head will be easier than changing the drive pinion.

This is such a perennial problem that - if I can't get at the pinion itself - I will design a sensor->computer->stepper-motor solution that'll allow our (cheap) mechanical speedo's to be calibrated to perfection across the entire speed range. The challenge that has stopped me doing this project so far is merely mechanical: connecting the stepper to the dial and finding a suitable cheap sender that can be connected to the cable end. No good having a sensor that fits on the gearbox because that doesn't solve the original problem. The computer bit in the middle is trivial.

 

One day, when I've cleared (or got bored with) my current and ever-growing list of projects, I'll get to work on it.

 

I could get the dial recalibrated. It's not cheap. And the recalibrator will probably build-in their own 3%-5% over-read margin of safety because they're not allowed to supply a speedo that under-reads (and their insurers would probably disown them if they did.) I'd rather have full control of the outcome.

[Spider]

If you can't get the Pot Joint off then another way in there is to take the Radiator out and come in from the top.

[mad4classics]

As an alternative, there are companies that can repair/ recalibrate mechanical speedo heads. Maybe worth an ask if they can do the work you require, taking out the speedo head will be easier than changing the drive pinion.

This is such a perennial problem that - if I can't get at the pinion itself - I will design a sensor->computer->stepper-motor solution that'll allow our (cheap) mechanical speedo's to be calibrated to perfection across the entire speed range. The challenge that has stopped me doing this project so far is merely mechanical: connecting the stepper to the dial and finding a suitable cheap sender that can be connected to the cable end. No good having a sensor that fits on the gearbox because that doesn't solve the original problem. The computer bit in the middle is trivial.

One day, when I've cleared (or got bored with) my current and ever-growing list of projects, I'll get to work on it.

I could get the dial recalibrated. It's not cheap. And the recalibrator will probably build-in their own 3%-5% over-read margin of safety because they're not allowed to supply a speedo that under-reads (and their insurers would probably disown them if they did.) I'd rather have full control of the outcome.

If you're feeling ambitious, then you might consider recalibrating the Speedo yourself.

I was inspired to tackle Speedo recalibration after reading TM9-1829A published in the US in 1944. It depicts a magnetizer and describes quite clearly the procedure required to recalibrate a Speedo.

As a winter project the recalibration took quite a while and many cups of tea, but in the end you can get it to read spot on.

Edited by mad4classics, 07 June 2025 - 07:08 PM.

[Spider]

If you're feeling ambitious, then you might consider recalibrating the Speedo yourself.

I was inspired to tackle Speedo recalibration after reading TM9-1829A published in the US in 1944. It depicts a magnetizer and describes quite clearly the procedure required to recalibrate a Speedo.

As a winter project the recalibration took quite a while and many cups of tea, but in the end you can get it to read spot on.

 

 

Yes, it is feasible to DIY on the Speedo, I've done a few, but the Odometer will still be out, need to swap gears around inside the speedo to sort that.
 

[alpder]

Yes, it is feasible to DIY on the Speedo, I've done a few, but the Odometer will still be out, need to swap gears around inside the speedo to sort that.
 

 

Not too worried about the odo (though it'll add another factor when calculating MPG and service intervals). Odd that this instrument is being fed almost exactly the turns the dial says it needs, yet is giving such wrong speeds. But who knows what's been done to it over the years. Maybe the odo is also wrong already... never thought to check. Hope it's not over-reading too: my already terrible mpg will take another hit.

 

Had a quick look under the wheel arch - the single(?) clamp screw is tantalisingly visible. Would be an easy job if there wasn't a driveshaft in the way. Sure a tool can be made/bodged to get in there. Don't really want to take the suspension apart.
 

[Spider]

 Don't really want to take the suspension apart.

 

 

If you can't get the Pot Joint off then another way in there is to take the Radiator out and come in from the top.

 

[Shooter63]

When I did this I ended up taking the rad out, I started by trying to get to it from below but soon got the hump with it. Obviously the rad out version can be a few minute job if you have the later brackets and no inner wing shroud, if you have that still in place, it's a different ball game. Even with the rad out I still undone the speedo cable using a long screwdriver through the rad slots in the inner wing. Putting it back together seemed to go really well, I fitted a new speedo cable at the same time.

Shooter

[sonscar]

Don't think too much,start early and keep going.It is not technically difficult and you are only doing it once.With tea breaks it will be less than a day.Steve..

[Ethel]

Child labour. They're good for more than chimneys.

[alpder]

Having checked the speedo against a GPS, thought I'd better check the odometer (against Back Country Navigator). Turns out the odo already under-reads by about 11%, before any attempt to mod the speedo gears. Which will, of course, make it even worse.

 

If the speedo is over by 16% and the odo is under by 11%... well, that's just weird. But it does mean that my hitherto somewhat depressing MPG figures are now a smidge more respectable. 37-ish instead of 33.5-ish.

 

Guess it also means that all Mini adventures up to now can retrospectively have 11% added to their adventurous-ness. The memories are suddenly even better than before.