Page 1 of 1
Speedometer inaccuracy.
Posted: Sun Oct 17, 2021 10:27 am
by will.broad
This has been talked about many times but I bought a GPS speedo to test against the Morris speedo. The Morris speedo was 10 mph over and got worse the faster I went.
I made a video to show it. I guess its the radial tyres that is causing it or maybe the speedo needs calibrating?
https://www.youtube.com/watch?v=Pbf7NAXPoTU
Re: Speedometer inaccuracy.
Posted: Sun Oct 17, 2021 10:56 am
by geoberni
Tyres, Wheels, Diff, Gearbox, anything that is different from the build standard ex-factory can, and will, affect the speed indication.
Is the car standard or have things been changed over the years, apart from the tyres....?
That seems a huge discrepancy just for tyres, but then you haven't said what size tyres are fitted.
There's been plenty of chat about the tyre options over the years.
Re: Speedometer inaccuracy.
Posted: Sun Oct 17, 2021 12:27 pm
by jagnut66
Hi,
If you need to get it recalibrated then I can recommend this company: http://www.jdo1.com/
They will explain what they need off you but basically what you have already been doing should be enough, namely a set of readings, Morris speedo compared to GPS speedo, say at 30mph, 50mph and 70mph.
Best wishes,
Mike.
Re: Speedometer inaccuracy.
Posted: Sun Oct 17, 2021 1:49 pm
by Sleeper
Can you give the TPM figure on the speedometer ?
( usually 4 digit number top right above the odometer )
John ;-)
Re: Speedometer inaccuracy.
Posted: Sun Oct 17, 2021 11:43 pm
by MCYorks
will.broad wrote: ↑Sun Oct 17, 2021 10:27 am
This has been talked about many times but I bought a GPS speedo to test against the Morris speedo. The Morris speedo was 10 mph over and got worse the faster I went.
Bear in mind, almost all speedometers over-read by some degree. They are not legally allowed to under-read, but they are allowed to over-read by up to 10% plus 6.25mph. So it could be reading almost 60 mph when your true speed is only 48 mph, and still be legal
That said, it won't help if you have the incorrect speedometer for the differential, gearbox or tyre combination fitted. As Geoberni has already pointed out.
There's a superb chart on the MMOC Potteries Branch website, to help identify speedometer types. It gives diff ratio, TPM, part numbers, etc.
Well worth a look.
http://potteries.mmoc.org.uk/ID/Speedo.htm
An interesting point. Most industrial measuring instruments are calibrated at least every 12 months, and sometimes more often. While a speedometer could have gone 50 years since fitting, without ever being calibrated or checked. However, we almost expect them to be reading correctly, when it's probably a miracle they work at all
