@flambonkscious @Dave The guy who services my car told me many moons ago that some vehicles are set to read slightly higher than they really are - as a safety thing.
¯\_(ツ)_/¯

[–] [email protected] 2 points 1 month ago (1 children)

You are absolutely right!

https://www.aa.co.nz/cars/motoring-blog/why-does-my-speedo-read-higher-than-road-side-speed-monitors/

[–] [email protected] 1 points 1 month ago

Oh great find!

[–] [email protected] 0 points 1 month ago (1 children)

Ooh that's interesting. I have seen this in almost every car I've ever driven past one of those things. There's actually one car that I remember borrowing one time that stands out in my mind as it matched the speed sign thing and that was unusual enough to still remember it years later.

I just assumed that since the speed is read internally (gear box or something?), as the mm if rubber come off the tire the speedo would over read more and more.

[–] [email protected] 0 points 1 month ago* (last edited 1 month ago) (1 children)

Maybe? Surely it's quite a few mm to get 3km/h over though?

Sounds probably like I need to learn math again... Eww!

Actually, surely tires aren't all the same size? Ours are recently installed and the measured difference hasn't changed.

[–] [email protected] 1 points 1 month ago (1 children)

Just found this reddit post. A comment says:

F1TV did a tech talk recently where they measured some tire wear. I don't know how many laps it ran, but the measurement was 0.6mm difference. If the pit limiter was calibrated to exactly 80kph for a new tire, that would equate to 79.87 kph on that worn one. I don't think that's enough for them to account for

I think 0.6mm isn't enough, but we can expand on that. A search shows a new tyre might have 8-9mm of tread. And legal minimum is 1.5mm, so that's 6.5mm or so difference, or about 10x the test above. Still, that would only make a difference of about 1km/h in the reading over the life of the tyre.

So seems 3km/h over is not explained by tyres. I guess F1 car tyres are smaller, but I'd think the impact would be smaller on larger tyres.

Actually, surely tires aren't all the same size? Ours are recently installed and the measured difference hasn't changed.

I thought you had to get the exact same sized tyres, because they had to fit the wheel.

Reddit seems to think it's calibrated to the factory installed wheels.

[–] [email protected] 2 points 1 month ago (1 children)

Thanks Google/Dave! - 0.6mm is a wild amount of wear but they are racing the shit of them...

Are you endlessly curious or something??

I did find this pdf from an nzta page that suggests there's a range of tire sizes available that a given wheel can accommodate (within their range of tolerance as well): https://www.lvvta.org.nz/documents/infosheets/LVVTA_Info_01-2009_V3_Tyre_Size_to_Wheel_Size_Compatibility_Guide.pdf

The 2nd page is a big honking table of different sizes, profiles and rim widths, so I gather there's a degree of anything goes and maybe next time we wear ours out, I'll try to be a little more attentive.. Maybe I can get some accurately sized ones (probably not worth it - getting pinged for going a few kmh over the speed limit would really suck!)

[–] [email protected] 2 points 1 month ago

Are you endlessly curious or something??

Haha I think I might be!

I did find this pdf from an nzta page that suggests there’s a range of tire sizes available that a given wheel can accommodate (within their range of tolerance as well)

I'm afraid I don't understand that table. There are no units listed, and I don't what what measurement the range is meant to represent. If the tyre width is 115 somethings and the profile is 80 somethings, then there's a range of 3-4 somethings. But what measurement is that talking about?

Given that the 6.5mm is about 1kph from the above calculations, I'm guessing similar tyre sizes probably still don't change it by more than a couple km/h.