Avocet

As others have said, I think the error would be low, unless falling off a cliff. They prefer straight (ish) line speeds, though. If you dove in a tight enough circle, the GPS speed might show zero, but I don't think it's possible to find a car with a tight enough turning circle. By and large they're good enough on a reasonably straight road or even one with shallow bends. Certainly more accurate than the speedo.

Yes, GPS is always more accurate. You might ask why manufacturers don't just use the GPS signal to power the speedometer? That's because if they did, they wouldn't be able to take full "manufacturer responsibility" for their type approval. Part of their speedometer "system" would be dependent on the American government (or Galileo or Glonass) and outside of their control.

20 is a pretty low speed though? What is it like at (say) 100 km/h?

Speedometer signals aren't always linear (in fact, they're usually NOT perfectly linear), so you can probably only configure it to be exactly accurate at one particular speed. (And of course, with one particular tyre, at one particular temperature, pressure and state of wear).

I sure am! It's pretty fishy that anything in Hansard (and thus UK-based) should talk in km/h for a start, but yeah, he's wrong. (Not just any MP, either, a member of the House of Lords)! The lowest speed that the ECE Reg 39 test is carried out at, is 40 km/h even at that speed, the noble lord's claim that manufacturers apply a tolerance of 5% + 10km/h, would put the vehicle out of compliance for both type approval and C&U. At a true 40 km/h it would display 52 km/h (which is above the maximum of 46.5 permitted by Reg 39). Either he was wrong, or there's a typo in Hansard and he meant 5% + 1 km/h. (Even then, he's wrong because all manufacturers don't apply the same tolerances in production. They can apply whatever they like, so long s they stay within the legal requirements!

Construction & Use is old UK-only legislation. Unlike type approval (which applies to the car "as manufactured", the C&U regs apply to the car "as used". In other words, if you buy a new car, the responsibility for speedometer accuracy rests with the manufacturer. However, you can go out and buy it a different sized set of wheels and tyres and by doing so, make the speedo inaccurate. Clearly, the manufacturer can't take responsibility for that, so it's down to you. That's why we need two sets of similar regulations. As you can see, C&U calls up the same ECE Reg 39 that I mentioned earlier. (It also gives the alternative option of complying with the old EC Directive, but that is no longer used these days by mainstream manufacturers and it's requirements for accuracy were exactly the same as in ECE Reg 39 anyway). However, I don't recognise the last bit about a production tolerance of 5%+10 km/h? That's definitely not true. It would be illegal for a start! At a true speed of (say) 20 km/h, the car's speedometer would read 21 km/h (5% of 20) Plus another 10 km/H = 31km/h! Whereas in fact, the type approval regulations would only allow a maximum of 10%+4 which would be 26 km/h at a true 20.

The regs I just quoted, (and posted a link to) a couple of posts above this one, you mean?🤣 Yeah, I know. (I type approve cars for a living)! The bit that was unusual, was linni's replacement Octavia speedometer being "deadly accurate". That's very rare. If a manufacturer makes a speedometer "deadly accurate" he runs the risk that one of them ends up under-reading by a tiny amount (which is not permitted under Reg 39). If the type approval authority that issued the approval catches him doing this, they can force a recall, withdraw his approval (so he can't register any more cars), and the European Commission can fine him up to €30,000 PER NON COMPLIANT CAR! So... When the type approval regulation tells you the speedo has to read anywhere between true speed and 10%+4 km/H ABOVE true speed, but under no circumstances ANY amount BELOW true speed... ...and you know the fines and penalties if they catch you having built a car outside of those limits... What would YOU do, if you were a car manufacturer?😉

Unusual! You would have thought that VAG would be on their best behaviour when it came to compliance, - after the emissions scandal!

Strictly speaking, for a mass produced car, it can over-read by 10% + 2.5 MPH, (At all practical speeds), but cannot under-read at all. Same in the EU. Paragraph 5.4 of ECE Reg 39: https://unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2018/R039r2e.pdf As a result, manufacturers always set them a few % high.

Possibly not quite the same thing but our 2017, 150 diesel DSG will "coast" when you lift off the throttle, but if you touch the brake whilst off the throttle, it will engage a lower gear and provide a bit of engine braking until the throttle is re-applied.

That's interesting. We'll be driving along, happily and then all of a sudden, we'll get the warning noise and a message telling us there's a fault in the e-call system and to contact a dealer. At the same time, the e-call light in the roof console turns red and we get a yellow warning triangle on the instrument display. Often, 5 miles or so later (and without doing anything with the e-call button or even stopping the car), the e-call light goes green again, and the dashboard warning triangle disappears!

As others have said, the modern cars do a whole lot more than just lights. Perhaps the single most critical thing, is that if it detects a trailer , it runs a different Electronic Stability Control programme - which among other things, will gave you some anti-snake protection. Fr us, it simply wasn't worth the risk. Not having the parking sensors going off all the time, not having the car's foglight reflection irritating you, and the blind spot detection sensors always being activated, was just icing on the cake!

Evening all. I finally managed to get under it this weekend and get a couple of photos and a video clip... 20230930_095900.mp4

Yes, it's very common. Our 2017 car was doing exactly the same thing. The e-Call (that's the generic nae for these devices, which are a legal requirement for manufacturers to fit, these days) module was replaced, with Skoda paying parts and us paying labour. It worked for several months, but now it randomly goes off again. They're a dreadful system! I'm told you can re-set it by pressing and holding the emergency call button for 30 seconds. I haven't tried it though, as it tends to re-set itself after a while anyway.

That certainly looks like it! Many thanks. Even that's £200! Also looks suspiciously like there's one at the other end too. I'd better check that...😒 Can't understand it. That car has led an absolutely charmed life (since we bought it, just coming up to 3 years old, at least)! Mrs. Avocet drives it like a mobility scooter - and not many miles, either.