1850 Auto, incorrect speedo reading

[Magenta Auto Sprint]

When i bought my 1850 auto a few years ago, it had 47,000 on the clock but had also had a rebuild/exchange auto box and torque converter.

The speedo has always been under reading, at 60 MPH it reads 45 and at 70 it reads 50.

Knowing that there are two speedo drive pinions, red has 22 teeth and green has 21 teeth. Today O took out a green pinion and fitted a red, no improvment, in fact I thin it is slower.

Any suggestions please?

Malcolm

[yorkshire_spam]

Does it under read by a % or a fixed amount? Looks like around 70 to 75% to me? Is it the correct speedo for the car?

If it's a % and it's the right speedo then I'd guess it's the wrong drive gear on the output shaft inside the tail housing on the gearbox, but this is unlikely.
If it's out by a fixed amount then there's a chance the needle on the speedo is on in the wrong place.

IIRC Green is the right drive pinion for an 1850 auto,

[new to this]

What size wheels and tyres are on the car ?

[xvivalve]

Was it an auto when it left the factory? If it's been converted it might have a non-automatic diff...

[Matt Cotton]

Hi Malcolm.

I once had my speedo recalibrated due to similar reasons.

I used https://www.speedycables.com/

If you decide to go down that route I can recommend them.

Best Wishes
MC

[Carledo]

Before you go Matt's route (and I thoroughly endorse Speedy Cables, they are a tad slow, but their work is worth the wait) may be worth just trying a different speedo! I have a couple here suitable for 1850 (not early font) you can have one for a lot less than a rebuild/recalibrate. As far as i'm aware, ALL Dolomite speedos from 1300 to Sprint are calibrated the same at 1000 TPM (turns per mile) so almost anything will do for a test article so long as the cable fitting matches, there are 2 types, screw on or clip on.

Steve

[MIG Wielder]

Hi Malcolm,

It's curious the speedometer is reading low.

My 1850 one is marked on the dial " Made in U.K. SNT 5368-OOB. 1000 ".

Its difficult to see how a change of axle would make it read about 72% low. Even a change from a 1300
Axle ( 4.11 ratio ) to an automatic 1850 ratio (3.27 ratio ) would only give you a 79.5% change. But it would read lower. If the change was done the other way the speedometer would read higher.

Couple of questions if you can help please.
I see from the figures you have given that the speedometer is reading about 72% of the correct value

Is the mileometer also reading about 72% of the correct reading ? If so its either the wrong speedometer head or the drive is slipping somewhere.
If the mileometer is correct its got to be the speed part of the instrument that is wrong. They can be recalibrated, but a 2nd hand one may be a better bet.

Tony.

[Magenta Auto Sprint]

Thank you all for some very good suggestions.

I am running 165/70 13 on sprint rims, I made a mistake when i bought the tyres, should be 175, Ill fit my 155/80 wheels off my 1300 and give that a try.

I checked the back axle today and it is a 3.27:1 unit.

I have fitted the green pinion back and did some data gathering.

speedo reading....GPS....Rev counter...% error....difference
.......20..............28.........1500...........40............8 mph
.......30..............41.........2300...........35..........11
.......40..............56.........2700...........40..........16
.......50..............67.........3500...........34..........17

(the table looks different in edit mode, hope you can see what I mean)

Figures are approximate, the wife wrote down the figures as I drove. My mums MK 4 auto Golf also reads 70 at 3500 revs,

The speedo is the early style and is marked up to 120, I do have a spare but the later style (thanks for the offer Steve), I had not thought about the odometer reading correctly, good point.

Malcolm

[xvivalve]

Question: does the speedo' needle hold a steady position, or does it seem to 'pulse'?

I had an inner speedo' cable break on me years back, but the ends of the break were rough enough for the rotation to transmit across the break most of the time but not all of the time, missing on a fairly regular pattern; my symptoms were not only the fluttering needle, but also reading low mph.

Probably not your problem as you've obviously disconnected the gearbox end recently as when I did that the short broken bit slid out.

I have a NOS early speedo' around somewhere with only 2 miles on the clock; can probably find a used one too...

[yorkshire_spam]

I am running 165/70 13 on sprint rims, I made a mistake when i bought the tyres, should be 175, Ill fit my 155/80 wheels off my 1300 and give that a try.

Online tyre comparison suggests that's a 3% difference in rolling circumference, so I don't think that's the source of the issue.
At that level of % error I'd start with a faulty speedo, assuming that the cable/drive pinion aren't obviously faulty.

[matt of the vivas]

Before you do anything else - drop the front end of the prop and make sure the nut that holds the output flange to the gearbox shaft is tight.
The flange is splined - so the only ill effect of this nut being loose is insufficient clamping force on the output drive for the Speedo which is immediately behind the flange and only secured by clamp pressure.
I've had this low reading Speedo problem on two different Automatics, in both cases tightening the flange nut was all that was needed to cure.
Matt.

[Magenta Auto Sprint]

Thanks Matt,

I'll do that as from the figures, the reading is not consistent.

Malcolm

[yorkshire_spam]

Before you do anything else - drop the front end of the prop and make sure the nut that holds the output flange to the gearbox shaft is tight.
The flange is splined - so the only ill effect of this nut being loose is insufficient clamping force on the output drive for the Speedo which is immediately behind the flange and only secured by clamp pressure.
I've had this low reading Speedo problem on two different Automatics, in both cases tightening the flange nut was all that was needed to cure.
Matt.

Good point! That possibility hadn't even occurred to me!

[triumphtoledo]

My 1500TC auto had a lazy speedo. The output flange nut was tight but managed to tighten it by another 2 turns using a long extension - problem solved!

[matt of the vivas]

Yep, seen this before several times - see my comment above.
Just read your article in the club mag. Your leaky flange nut is because you need some sealant on the splines - it would have had some applied originally. Loctite make a good liquid flange sealant, brush some on the splines before you refit the flange.
Matt.