How's speedo accuracy?

[Joe Boatman]

At 31 mph in my 2021 MG5, the nearest grinning radar speed sign displays 30 mph!

 

[tsedge]

Mine is spot on, as close to exactly the GPS speed as I can measure at all different speed limits.

This was a big change from my last car, a 2018 Skoda Karoq, which indicated 76mph at an actual 70mph.

Worth noting that the MG4's efficiency is better than some competitors simply due to the fact that some competing cars over-read significantly which exaggerates mi/kWh.

 

[decrep]

Worth noting that the MG4's efficiency is better than some competitors simply due to the fact that some competing cars over-read significantly which exaggerates mi/kWh.

Although most of my previous cars exaggerate speed up to 8% the distance has always been much closer, unless I've changed tire profile.

 

[tsedge]

Although most of my previous cars exaggerate speed up to 8% the distance has always been much closer, unless I've changed tire profile.

That's interesting. I have never measured distance independently, I assumed it was always driven by the speedo. How do these cars get one right but not the other?

I am assuming that they deliberately overread the speed but not the distance?

 

[decrep]

I am assuming that they deliberately overread the speed but not the distance?

That's my assumption. Here the rules are, "mustn't be over but can be 8% under".
Both fed from the gearbox take off, but to be on the safe side, the speedo is calibrated to overread.

 

[Paulie68]

That's my assumption. Here the rules are, "mustn't be over but can be 8% under".
Both fed from the gearbox take off, but to be on the safe side, the speedo is calibrated to overread.

So is that 8% just for mileage recording?, as with speed shown, they're not allowed to under read at all, but can over-read by up to 10% plus 6.25 mph in the UK

 

[decrep]

distance is normally very accurate, just varies a bit with tire wear.

 

[smokie]

... can over-read by up to 10% plus 6.25 mph in the UK

That would put you at nearly 40 in a 30.

If you're talking about the speed at which you can get prosecuted, the NPCC (was ACPO) guidelines (which most forces follow) is 10% + 2mph.

I understood recently that some forces don't follow those guidelines religiously and some even sometimes use zero tolerance. But most still follow the guidance apparently.

 

[Paulie68]

That would put you at nearly 40 in a 30.

If you're talking about the speed at which you can get prosecuted, the NPCC (was ACPO) guidelines (which most forces follow) is 10% + 2mph.

I understood recently that some forces don't follow those guidelines religiously and some even sometimes use zero tolerance. But most still follow the guidance apparently.

No, you've mis-understood! We are talking about "over reading" speedometers, so it's showing more than what you're travelling at, not less! So at the maximum allowable error, you'd be travelling at around 22mph whilst the car would be showing 30mph.

We're discussing allowable error from the speedometer & mileage recorder in the car, nothing to do with speeding offense prosecution limits!!