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Today I confirmed what I'd long suspected: my NC's speedo is running exactly 10kph slower than the real speed: ie, an indicated 100kph is actually only 90kph, etc etc.
I suspect that this may be because I bought the car with 45 section tyres and have replaced them with the same profile, and maybe originally it was a different profile??? Anyway, I'm happy with the tyres, so how do I recalibrate the speedo???

What width and diameter wheel?
Sidewall height is dependant on these so would be helpful to know.

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Hey, it's got 205/45 ZR 17 on the sidewall.....

That's the correct size.
Someone on here (Lokeil maybe?) Has used one of these:
https://www.jaycar.com.au/corrector-speedo-module/p/AA0376

See the MCM video about them:

Funny I just watched that, love the Mighty Car Mods boys..

Confirmed by an App or something?

Correct, we downloaded an Android app and ran it on the highway on the way to Goulburn this morning. Supposedly it links to the GPS satellites and gives exact speed...

I would double check by using a difft App.

Mine over read by about 6% with standard 205/45 tyres. I now (for the last 5 years) run 215/45 and that size brings the speedo over read down to about 2%. However, the odometer now under reads by about 4%.

Your 10% ish over read seems a bit too big but, not all tyres of the same section are created equal, so that may be contributing as well.

As an aside, our Mazda6 and BMW E82 both over read by about 5-6% with standard tyres.

Someone may have changed the diff to a different ratio at some point.
Also, I second what Bruce said. Worth a check with someone else's phone and a different app.

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