Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Speedometer Calibration

Some time ago I mentioned that a method of correcting the built in inaccuracy of the M3W speedometer has been found. The details of how to do this are in an article in the Morgan Three Wheeler Club "Bulletin" magazine and a link to that is here: https://www.mtwc.co.uk/wp-content/uploads/2020/01/Bulletin-December-2019-speedo-calibration.pdf. This is free to view, even if you are not a MTWC member, although there is a lot more information about M3W's available to members if you look at the list above the article. The speedo correction is easy to do, once you know how and is essential if you plan on using lower profile motorcycle tyres, as the speedometer error will be in excess of the 10% over read error permitted for UK MOT's. It's worth doing even if you retain the standard size tyres, an error of about 6% is just not useful. As you will see, the most difficult part is locating the calibration plug.

Excellent info! Thanks a lot Planenut!

Thanks! I have the December MTWC Bulletin sitting on my "to-read" magazine pile in the, ahem, reading room... but have not opened it yet. I'd also suggest folks consider joining the MTWC. They are making a good effort at including info and articles on 5-speeders.

The actual VDO instructions, as pointed-to in the article, are an incredible find, link re-posted here for future reference: VDO Installation Instructions

It is news to me that MOTs cover speedos - never used to be the case but I am an old fart with too many memories and not enough up to date facts!
In any case, how do they carry out the test?

I guess they could do something on the brake test rollers but it would be very rudimentary.

I was also not aware that they checked the speedo accuracy but IIRC they do check to see if it is working.

After a bit of research I’ve found this from the MOT Inspection Manual

https://www.gov.uk/guidance/mot-ins...ht-commercial-vehicles/7-other-equipment

7.8. Speedometer
You must check the speedometer of vehicles first used on or after 1 October 1937 with a maximum speed above 25mph. You do not need to check Class 3 vehicles.

If a road test is needed, for example to carry out a decelerometer test, you must check whilst driving that the speedometer is working.

If a road test is not necessary, you should only reject a speedometer if it’s clearly not working.

Speedometers do not need to be lit on:

vehicles with no front or rear position lamps
vehicles that have front or rear positions lamps that are permanently disconnected, painted over or masked
You can accept a tachograph as an alternative to a speedometer if it satisfies the requirements of this inspection.

Defect
(a) Speedometer not fitted where one is required .... Category .. Major

(b) Speedometer:
(i) operation impaired ..... Category .. Minor
(ii) not working .................................. Major

(c) Speedometer:
(i) not sufficiently illuminated ............. Minor
(ii) not illuminated ............................... Major

Speedometers are permitted to over-read by up to 10% (I think), but it is illegal for them to under-read. The MOT tests whether they are working at all and could pick up an under-read as a defect (but I doubt that it would be spotted).

Thanks for posting this! It took less than 10 minutes bonnet-off to bonnet-on, and it will solve an irritating situation!

Is this the right plug? Late model year

Interestingly my Aero over-reads as well, and if I recall is a very similar procedure (once you are into the speedo settings).
I shall have to see how accurate (or not it seems) the M3W one is!

Thanks for posting.