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Speedo Reading Low
Posted: Thu Jul 29, 2021 4:16 pm
by johnhannam
I have a 948cc Minor convertible and my speedo now reads about 15mph low. I have recently replaced the finned gearbox with another second hand finned gearbox when the problem started. I have tried replacing the perfectly good speedo and the problem persists. Why should a gearbox swap for two seemingly similar boxes result in this aberration? Have I missed something?
Regards,
John
Re: Speedo Reading Low
Posted: Thu Jul 29, 2021 6:07 pm
by oliver90owner
Different parts in the gearbox is the reason. Speedo drives are different for different rear axle ratios?
Re: Speedo Reading Low
Posted: Sat Jul 31, 2021 1:49 pm
by Declan_Burns
John,
If you can't get it sorted I can build you a speedo corrector gearbox. It is not possible to upload photos at the moment for some reason or other.
Regards
Declan
Re: Speedo Reading Low
Posted: Thu Aug 05, 2021 3:33 pm
by johnhannam
Hi oliver90owner,
I am not an expert but,
1] the new speedo has the same serial number as the one I replaced {and the same facia}.
2]My replacement gearbox was replaced with another finned gearbox from another 948cc Minor. Is it possible that different finned gearboxes have different speedo drive ratios? So for example a 948 cc van and a 948 convertible have different speedo drive ratios?
3]I have not changed my rear axle.
The mystery deepens.
Regards,
John Hannam
Re: Speedo Reading Low
Posted: Thu Aug 05, 2021 5:58 pm
by philthehill
Firstly your gearbox - a Minor with a 948cc engine originally had a smooth case gearbox.
The gearbox speedo gear on the rear of the 3rd motion shaft for both for smooth and ribbed gearbox cases is Pt No: 2A3371 (MOSS).
The pinion speedo gear again for both smooth and ribbed gearbox cases is Pt No; 2A3420 (MOSS).
Whilst you have replaced the speedo like for like the speedo you replaced may not have been the correct speedo in the first place.
Again whilst you may not have replaced the rear axle/differential it may have been changed in the past.
A photo of the front of the differential and a photo of the rear of the axle would be very helpful. The differential ratio is stamped on the top of the differential mount flange so you can determine the diff ratio without having to strip anything.
What size tyres do you have fitted?
With things like this never take anything at face value.
Phil