over reading speedo

[timberman2004]

1971 Trav, in pretty good order, and i believe has not been messed around with previously...but who knows

However the speedo seems to be over reading by about 20%, ie 50 MPH on my phone speed app = 60 on the speedo. this discrepancy is also reflected in the speedo odometer reading, so calculating the correct MPG is a bit irritating back calculating with my abacus and slide rule

the needle is steady and progressive, not jerky ...Any hints ?

[Declan_Burns]

What is the TPM on the speedo?
Have you a standard four speed gearbox fitted?
What is the differential ratio?
Regards
Declan

[liammonty]

If the discrepancy is definitely reflected in the odometer reading, I would be checking tyre sizes and also the speedo TPM, as Declan has suggested. It's quite normal for the speedos to overread as you describe even if they are the correct ones (bear in mind that all modern cares do too, in my experience between 5 and 10%) but on Minors I've found that the inaccuracy tends to get greater as the years go by, due to a weakening spring in the speedo mechanism, which allows the speedo to read higher than it should as the years go by and the resistance of the spring gets lower. However, this isn't accompanied by a change in the accuracy of the odometer, which is why I wonder if you've got the wrong unit or tyres fitted.

[will.broad]

Mine is the same, I'm thinking its to do with the tyre size. Did Morris Minors have crossply tyres originally as they are a bigger tyre than radials and that would explain the difference.

[Declan_Burns]

If you can't get it sorted I can build you a speedo corrector gearbox that sits on the back of the speedo. I have built loads of them over the years for differential conversions mainly for MG but the speedo threads are the same (M12x1mm) on the Minor.
Regards
Declan

[liammonty]

Mine is the same, I'm thinking its to do with the tyre size. Did Morris Minors have crossply tyres originally as they are a bigger tyre than radials and that would explain the difference.

Yes - I think that's definitely a part of it, as the rolling radius of the radials is quite a bit less then the crossplies

[myoldjalopy]

It only over-reads? I should be so lucky - mine under-reads and then over-reads repeatedly every couple of seconds....and the oscillations get wilder the faster I go!

[timberman2004]

most helpful chaps,

problem with the Morris is its all too easy to get anal about it ...every little sound that wasn't there yesterday, tweaking and fettling
kids refer to it as Dad's Meccano set ....that's them written straight out of the Will !

the speedo is def a 1408 TPM (turns per minute I guess?) & visual layout exactly like Declan's pic
Std 4 speed gear box, but don't know about diff ratio, sort of just what they sold me.... it really doesn't much like going over 60 real speed (or 70 + on the speedo) all a bit vibey, but then as an old lady & deserving a bit of machine sympathy, i don't push it too hard

I'll check the tyres when it's light
Advantage of over reading is I'm safely under for the speed cameras, but Declan's mini gearbox does look very fine

[Myrtles Man]

Turns per mile.

[Sleeper]

a 1548 ( or thereabouts ) speedo would give you a correct reading, ( I think the nearest standard is 1550 ).

John ;-)

[timberman2004]

The tyres are Bridgestone 145 80R 14, so ideal tyre pressure? perhaps that might help a shade ?

[Nickol]

This may be useful if it applies.

When I did my LHD conversion, having the speedo read mainly in MPH and the odometer also in Miles was pretty impractical when all around is in km. I got hold of a speedo from a Mini which was an "800" version. Having installed it, it recorded alarmingly high speeds, even worse than yours. I seem to remember it showing 80kmh when doing actually 50kmh.

A tip from somewhere informed me to remove the pointer and reset it by trial an error. From memory this involves firstly removing the glass of course. The pointer gets automatically pulled back to zero by the spring mounted behind it and thus rests against the stop. By removing the pointer and holding the spring back, you can reset it in a new position. I had to do this 2 or 3 times before a satisfactory result was obtained. Now it only slightly overreads at 80kmh, which is what most speedo do anyway.

[oliver90owner]

There is a UK law regarding speed indication for vehicles. +10%, -0%.

[liammonty]

Ideal tyre and the perfect (manufacturer recommended) size for your wheels. However, as will.broad mentioned above, the 'height' of the radial, and subsequently, their rolling radius, is less than the crossplies that were fitted (which I believe had a 100% aspect ratio vs. the 80% of the radials). So that in itself will lead to a little overreading. Correct pressures may help a tiny bit, but unless they're flat, I don't think you'll get a noticeable difference.

Rest assured that you're not alone with this issue - I've had a Series MM, a Series II, one 948cc 1000 and 4 1098cc 1000s, and each and every one has had an over-reading speedo. It's 'cured' in one of my current cars (a 948) because I fitted a higher 4.22:1 diff ratio in place of the original 4.55:1 and didn't change the speedo accordingly!

[Declan_Burns]

Liam,
Exactly what I did. I changed the diff from 4.22 to 3.9 and the speedo was then spot on.
Regards
Declan

[liammonty]

Liam,
Exactly what I did. I changed the diff from 4.22 to 3.9 and the speedo was then spot on.
Regards
Declan

It's an additional benefit of a diff ratio change, isn't it Declan!! I like the sound of your reduction gearboxes - that's a really neat solution

[timberman2004]

I'd hazard that Declan's little gear box might be a shade cheaper and certainly an easier fit than finding the right unit and swapping out a diff

[will.broad]

I am thinking of getting one of these GPS Speedometer's.

https://www.ebay.co.uk/itm/JJC-GPS-Digi ... %7Ciid%3A1

[oliver90owner]

I am thinking of getting one of these GPS Speedometer's.

https://www.ebay.co.uk/itm/JJC-GPS-Digi ... %7Ciid%3A1

Expensive? (Nearly) all sat navs do the same, except not such a large display? They, too, may be inaccurate to some degree - except on the flat? Speeding down-hill (over a cliff edge?) would be particularly flawed, I would suggest.

[Sleeper]

My satnav doesn't like cloudy days...is it the same with the speedometers?

John ;-)