Speedo accuracy and different sized tyres

Hi,

I have recently changed from 205/70 R15 to 215/75 R15, the difference in circumference is about 5%, so I expected my indicated speed to drop by 5%, my first thought was speed cameras/vans.

I have a Android head unit with GPS, so thought I would compare the two GPS speed vs indicated speed (the Android head unit will show both, side by side).  There was almost no difference.  Now I know for GPS to be accurate, you have to be on a straight, level road and travelling at the same speed for a little way for the time average to balance out, but I expected the indicated speed would be about 5% less than actual.

So I checked my other car which is running on standard profile tyres and the GPS is about 8% less than the indicated speed.

I then found this article
www.startrescue.co.uk/breakdown-cover/mo...te-is-my-speedometer
'Under UK law - which is based on a European Union standard -  UK speedometer regulations also outline how accurate speedometers must be.The regulation states that speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph.So if you’re going 40mph, your speedometer may read up to 50.25mph - but it can never read less than 40mph. In order to stay within the law, carmakers calibrate their speedometers to slightly overreport their vehicles' speeds.'

So my guess would be that my Jimny over-stated the speed by about 5% before the tyre change and my other car is over-stating by about 8%, at 60 or 70 that is quite a bit and perhaps it explains why some people hurtle past even when I am slightly over the speed limit.

I guess the moral of this is if you are thinking of changing tyre or wheel size check the speedo against the GPS in your phone before thinking about gear reductions in the speedo drive, at least you will know where you are before you fiddle.

To check speed against GPS, drive at a steady speed on a straight level road for at least 10 seconds before making the comparison otherwise the GPS will nearly always under-read speed.

Robin

Fitted over size tyres on FL2 years ago to get 20mm+ ground clearance.
checked speedo reading against Iphone andriod and a tom tom. It put the speedo bang on.
Car makers do this so they can't be sued for your speeding.
New tyres have 5+mm more than a badly worn tyre. This will alter the speedo reading. That's one reason why manufactures build in the " you can't sue us if you speed"
you can try this one at home kids.