MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - speedo problem

My speedo has just started not reading over 20mph it shows correct speed up to 10mph then slowly reaches 20mph when car doing 50mph via gps speedo on phone. the odometer reading distance correctly it seems to have something stopping the needle moving correctly sn6142-09 model
mark heyworth

I didn't know the answer to your question Mark so asked Chatgpt.
This is what it came up with

✅ Likely Causes
1. Dragging or sticky speedometer needle mechanism (very common)

Inside the speedometer, the pointer is moved by a small aluminium “drag cup” interacting magnetically with a spinning magnet.
Over time, the following can happen:

Old dried grease becomes sticky.
The tiny spindle/pivot the needle rotates on gets gummy.
The hairspring that returns the needle gets stiff, contaminated, or out of calibration.

Symptom:
Needle works at low speeds but cannot swing freely as speed increases → it “hangs” and tops out around 20 mph.
✔ This matches your observation perfectly.

2. Magnet weakening or slipping on the shaft (less common)

If the rotating magnet inside the gauge is weak, it won’t pull the needle as strongly as it should.
You’d still get some low-speed movement but not proportional at higher speed.

3. Internal drag from the odometer gear train

The odometer could be running correctly but still producing extra internal friction in the mechanism that couples the drag cup to the needle.

❌ Causes that are unlikely:

Speedo cable (because your odometer works and the needle does move smoothly at low speed).

Gearbox drive pinion (same reason).

Calibration error (calibration errors don’t cause a “ceiling” at 20 mph).

✅ What you can do
A. Quick external checks (safe, basic)

Make sure the cable isn’t kinked and is screwed firmly into the speedometer.

Lightly tap the gauge bezel while driving slowly — if the needle jumps, the spindle is sticking.

B. What actually fixes it

The speedometer head needs to be disassembled, cleaned, and re-lubricated, and the spindle pivot freed.
This is a delicate job involving:

Removing the bezel and glass

Extracting the pointer

Cleaning the drag cup shaft and pivots

Re-oiling with very light instrument oil

Checking hairspring tension

⚠️ Because the mechanism is fragile and involves small precision parts, most owners send these Smiths units to a specialist.

🛠 Recommended repair path

You’ll want a speedometer repair shop familiar with Smiths instruments.
Common shops people use include:

JDO Instruments (UK)

Speedy Cables (UK)

Nisonger (US, for Smiths)

They typically:

Clean & lubricate pivots

Re-calibrate the magnet

Replace worn jewel bearings

Re-face/restore if needed

Cost is usually £70–£140 depending on work.

If you want to try a gentle DIY diagnosis

I can walk you through non-destructive checks, such as removing the speedometer from the dash and testing the input shaft by hand to confirm internal drag.

Just tell me what tools you have available and whether you’re comfortable removing the gauge from the dash.

It seems that number 1 describes your symptoms?
I can ask for a step by step repair process if that would help?
Jeremy MkIII

Point 1 would be my choice for initial investigation.

If the bezel and glass can be removed then the guts can be removed and apply a small amount of a very light oil to the spindle connected to the drag cup and needle where it passes through the die cast upper frame. The clearance is very small so use a very light oil if you have some, the same can be done for the input shaft which has the rotating magnet. It's basically an eddy current brake. Once the oil has had a bit of time to penetrate then exercise the assembly by rotating the aluminium cup through its normal range.

Dismantling them further isn't difficult but would require recalibration.
David Billington

Chatgpt.
Jeremey, that is a pretty impressive response from Chatgpt. Nuanced to the specific problem and well composed.
Is this unedited version or did you have to clean it up? And how much detail did you input to get that level and accuracy of response?
GuyW

Assuming an original gearbox. I'd start by looking at the inner of the speedo cable before anything else. Very quick qnd simple job.

Just undo from the back of the speedometer, pull the inner cable out of the housing and see if it's dry or damaged in any way.


As for ChatGPT.

I asked, how often is chatgp right?

Thus is the answer, from ChatGPT.🤣.

"The accuracy of ChatGPT
varies significantly depending on the topic and model version, and it is not always right.

While the latest models (like GPT-4o and GPT-5) achieve high accuracy rates (88-94%) on general knowledge and standardized tests, they still "hallucinate" or make up information occasionally, especially in specialized domains. "

Hallucinations: The model can generate information that sounds highly confident and plausible but is entirely false

OpenAI and experts recommend treating ChatGPT as a powerful assistant, not an infallible authority.

In summary, ChatGPT is a highly capable tool with impressive accuracy in many areas, but human oversight and verification remain essential to mitigate the risk of acting on incorrect information"





anamnesis

Guy,

I cut and pasted Mark's original post into ChatGPT and that's what it came up with. It asked if I wanted a detailed description of how to do the work and that's also pretty useful. (Can post if anyone's interested).

You're right Anam, it does hallucinate. Rather than say it doesn't know something, it'll make it up.
If I've understood correctly, it relies on data which has been posted online rather than being able to work it out for itself, so its response to this question is likely to include information which has been posted on here at some time or other.
It's a very useful tool but it's not infallible.
Jeremy MkIII

Yep Jeremy. But as much as I found it funny, how long, before 'it' has so much data banked at its instant call, that in milliseconds or less, it is able to truly simulate rational thought?

Isn't that what we do? We search our remembered experience, sift it, contrast and compare, and come up with an answer or solution. Sometimes right, and sometimes wrong, we improve as we go along.

Not far off perhaps, everything you think do and say, will be from the machine you consulted today.

There could be a song like that. 😉






anamnesis

You worried about take-over, Anamn?
Should I ask cHAtGPT for advice on modifying my Sprite to make a drain down system for the coolant?
GuyW

Ask it about oil sucking. 🤣🤣🤣.
anamnesis

Actually that would be interesting Anam.
Have you kept a synopsis of the fault to copy in to it?
Jeremy MkIII

The briefest synopsis was about a 1000 posts. 🤣🤣.

Worth a try, just for fun. 😄

anamnesis

I typed,

"oil sucking engine breather in mg midgets"

Response

"Oil sucking in Midgets, especially 1275cc models, often means a clogged crankcase breather system, particularly the oil separator in the timing cover or tappet cover, which builds pressure, forces oil out, and causes blue smoke/oil loss; cleaning or modifying these separators (sometimes with steel wool or larger holes) and ensuring proper closed-crankcase ventilation with a vented oil cap and hose to the carb/manifold is key to resolving it"

And then a whole load of other stuff follows, where anything possible as a cause is mentioned.

Kind of the kitchen sink approach. Lol.

Which actually, is how all the engine breather oil sucking threads ended up. 🤣🤣🤣.

I wonder if Spock had as much trouble? 😉

https://youtube.com/shorts/nTCKL348xg0?si=Gq7Vqg5w8PjjTiVT



anamnesis

AI…

The Telegraph reports that the Sandie Peggie tribunal judgment contained a completely invented quote, possibly even an AI hallucination.


Dave O'Neill 2

So AI has learnt how to mimic journalists and/or politicians?🤔
Jeremy MkIII

Had time today to take it out and have a look inside it found the return spring on the needle was broken re attached it and works fine now
mark heyworth

This thread was discussed between 10/12/2025 and 21/12/2025

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.