I had a loaner car (a Peugeot) last week while my clubman was in for repair. Whilst driving the loaner with my GPS unit and also watching the local speed radar displays (i.e. those solar powered units that show how fast you are going when entering a village), I noticed that the spedometer of the Peugeot, my GPS and the village radars were all in agreement.

Previously I had noticed a rather large discrepancy with my Clubman.

After I got the Clubby back, I did some further testing. It turns out that the analog spedometer on the Clubman is overestimating the speed of the car by about 8-9km/hr and the digital display is overestimating by about 6-7km/hr. In other words, when the big analog display says I'm doing 109km/hr, I'm actually only doing 100km/hr.

While this is helpful for avoiding speeding tickets, I can't help but be a little disturbed by this. The car was ordered from the factory with 15" rims and I'm still on the stock rims and tires, so this inaccuracy appears to have come direct from the factory.

Is there a way to have this adjusted so it's actually accurate? Has anyone else noticed this problem?

I think theres a 10% error margin allowed on car speedo readouts, normally they read faster than you are travelling for good reason as you say, speeding.
Wheel size and tyre size changes can affect this and speedo's used to be normally calibrated as turns per mile of the speedo cable, but in these day of digital speedo's I would have though they could get it more of less spot on.
But the error you say I would suggest is within normal allowances, you will be glad it reads fast when youve just missed the mobile speed cam though.

I think theres a 10% error margin allowed on car speedo readouts
I think that's mixing up two separate ideas:

- Traditionally (as in since the days of steam cars) the British police allowed a 10% margin for speedo over-reading before prosecuting for speeding. Nowadays the ACPO guidelines are 10%+2mph (ie, 35 in a 30, 79 in a 70) but those are only guidelines - I think it was Teeside that at one time had a zero tolerance policy where 31 in a 30 got you booked.

- I read once that EC regulations on speedos require that two standard deviations from the mean to be at or above the true speed, which is an intelligent way of defining the limit. That would mean that about 2% of cars produced could have a speedo reading below the true speed before the manufacturer was getting it wrong.

I don't believe there is any upper limit, anywhere, on how much a speedo over-reads. Most Japanese motorcycles demonstrate this by having speedos that over-read by huge amounts - reading more than 20mph over the true speed isn't unknown.

Thanks for this.

Actually, I'm surprised the error margin is allowed to be so high. I would have imagined 5% was more than sufficient. I understand as well that tire wear can contribute to the inaccuracy, but for me 10% is rather glaringly large and seems excessive.

Is it possible to have a dealership recalibrate this or is it fixed for the life of a car? I can't help but imagining what would happen to the inaccuracy if I were to swap out the 15" for 17" or something. In such a case there must be a way to apply an adjustment factor?

I guess I'm also struggling with the idea that the Peugeot was basically spot on, while my Mini was kind of "out in left field" as it were.

I was always led to believe they can be up to 3% innacurate, but they are calibrated at 15 degrees so will be innacurate for any temperatures above and below that

I have noticed this too mini nav shows 56mph but mini says 60mph - which do I trust:confused:

Thanks for this.

I can't help but imagining what would happen to the inaccuracy if I were to swap out the 15" for 17" or something.



I just changed my 15's to 17's with low profile tyres and i can't really say i have noticed a difference, if its out i would say maybe 1-2mph but as i say, i really can't notice a difference.

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