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I have recently upgraded to 17" rims from the original 16" ones on my IS220d. On a recent trip to Leeds I noticed, where the distance had always been 177 miles door to door before, it was now 170 miles. I do realise this is to do with the slightly bigger turning circumference, approx 6cm, in the rear 245/45 R17 wheel/tyres in comparison to the original (ugly) 205/55 R16s.

My question is which is the more accurate?

I am thinking possibly the 17" rim mileage, as I also noticed my speedometer speed more closely matched the speed on my TomTom compared to before, where it always seemed that tad faster.

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the speedo is designed to read over, but the mileage should be accurate. If the speedo is now more accurate then your mileage may actually be under

So is the TomTom speed reading the most accurate indication of my actual speed? As it's measured by the GP Satellites.

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the speedo is designed to read over, but the mileage should be accurate. If the speedo is now more accurate then your mileage may actually be under

So is the TomTom speed reading the most accurate indication of my actual speed? As it's measured by the GP Satellites.

The two wheel/tyre combinations have exactly the same rolling circumference. The TomTom would be more accurate except it measures assuming you travel in straight lines between its measuring points and there is no update delay. The speedo is subject to analog linearity errors whereas the odometer is a simple count of revolutions of the wheels ratioed according to the calculated rolling circumference - so is subject to error which depend on tyre pressure etc. plus the circumference increases slightly with speed and tyre temperature.

None are capable of accuracies better than about 2% - 3% with the speedo worse than that at some speeds and biassed for the errors to be positive i.e. indicate faster than true speed. There is no easy way without laboratory equipment to show which gives the most accurate result.

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