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Speedo Calibration

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doxendine Donnie Oxendine
Huntingburg, IN, USA   USA
I've just completed my 1275, rib case, 3.9 differential swap into a 58 Morris Minor. I have no idea of how far off the speedometer will be. I haven't driven it yet as I'm installing new carpet. Can anyone clue me into a fix?

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Sl55per John Clegg
Manchester, Greater Manchester, UK   GBR
I think a 1029 TPM would be a good start..not an off the shelf item .

Best to refit yours and see how far out the odometer is , also the speedometer then return with those figures and the tpm marking on yours...

Johnwinking smiley



Edited 1 time(s). Last edit at 2019-01-09 12:18 AM by Sl55per.

0123 Mike D
Biddulph, Staffs, UK   GBR

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gjoakes Silver Member Greg Oakes
Cornfield, IL, USA   USA
1958 Morris Minor Traveller "Bridget"
1972 MG MGB "Buttermilk"
I have that same combination (original '58 speedometer and gearbox is from a '70 Midget) and with Firestone F560 155R14 tires I find that at speeds from 35 MPH or so up to 60-65 MPH it reads a roughly consistent 5 MPH slow (verified with GPS).

Heck, I can do that calculation in my head so I just drive it as is and adjust my speed accordingly. Somehow it adds to the experience.

66jalopy Avatar
66jalopy Silver Member Phillip Jolliffe
Lake City, FL, USA   USA
I use my Garmin GPS or an app on my phone. Mine is off by about 10mph

Sl55per John Clegg
Manchester, Greater Manchester, UK   GBR
My Odometer is true to 1% now,but the speedometer is reading 10% slow , when I get some time I'm going to try re-magnetising the permanent magnet and degaussing it down bit by bit to regain the accuracy..

As for summer I'm going to fit LP917 s with larger tyres so will have to do it all over again,new TPM gearings and altering the magnet ( albeit using two speedometer heads , one for summer , one for winter )

Johnwinking smiley

0123 Mike D
Biddulph, Staffs, UK   GBR
Summat not quite right there Greg.

5mph fast at 20mph should be
10mph fast at 40mph
15mph fast at 60mph.

0123 Mike D
Biddulph, Staffs, UK   GBR
John in Eugene Avatar
John in Eugene Platinum Member John Quilter
Eugene, OR, USA   USA
Nissonger Instruments, the official Smiths agents on Long Island New York, have a process to take measurements and have them re calibrate the speedo. They did mine on my MGTD a while back (it was changed from 5:12 gearing to 4:55) and it came back dead accurate. Basically you measure out 52.8 feet (100th of a mile) in a straight line an a flat surface, roll the car forward that distance and count accurately the number of turns of the speedo cable. Do it three times for accuracy, supply them this info on their form and they reset your unit to read accurately in speed and distance.

John F. Quilter
Eugene, Oregon USA

In reply to # 34178 by doxendine I've just completed my 1275, rib case, 3.9 differential swap into a 58 Morris Minor. I have no idea of how far off the speedometer will be. I haven't driven it yet as I'm installing new carpet. Can anyone clue me into a fix?

gjoakes Silver Member Greg Oakes
Cornfield, IL, USA   USA
1958 Morris Minor Traveller "Bridget"
1972 MG MGB "Buttermilk"
Mike,

Couldn't say what mine is at 20MPH, I've never checked it there. But I have checked it at 35-40MPH where it is a little less than -5MPH error and at 60MPH where it is a little more than -5MPH error. So I call it a constant -5MPH regardless of speed. I'm not particulary concerned about speeding tickets doing it this way. As I said, it's part of the charm of driving a Morris Minor.

0123 Mike D
Biddulph, Staffs, UK   GBR
Lets say 37.5mph and 4.8mph out,
the error at 60 mph should be 4.8x60/37.5 Greg.

Which suggests your speedo needs an overhaul
as well as recalibrating.

0123 Mike D
Biddulph, Staffs, UK   GBR
The way to go if you want an accurate speedo thumbs up

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