When driving on the dual carrageway at an indicated 70 mph along with everyone else, we all do down here , of course, !! I always seem to be traveling 1 or 2 mph more, is my speedo correct, as I’m running 22inch tyres. Good question for Tyrefitter perhaps?

All tyres fitted to an RRS are the same diameter whether on 22 inch wheels, 21s or 20s. The speedo on an RRS is about 4-5% optimistic. For instance when I see 105kph on my speedo I am actually doing 100kph. RRS P460e
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Last edited by rangiejools on Tue Dec 04 2018 11:57am. Edited 1 time in total

There is a 10% high margin of error permitted on uncalibrated speedos so at 70 speed limit you can be up to + 7 mph out. You can get an indication of errors using a phone app route guidance program online for your phone - one that shows your speed and you can compare the readings. Don’t use the in car nav system as it is an average not an instant reading

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Last edited by Tim in Scotland on Wed Dec 05 2018 9:09am. Edited 1 time in total

As Tim says, ff you download a GPS speedo app you will get a better picture of your actual speed. On mine 74mph is 70mph. 95mph is 89mph (tested on a private road of course). SOLD - MY16 SDV6 Autobiography Dynamic - Fire Engine Red, Fixed contrast pan roof, TV, Dual Screen, 5+2, Privacy, Noble paddles.

I have a Road Angel in my car which is GPS based. At 70mph on Road Angel the RRS speedo is showing 74. I never look at the car speedo now, always Road Angel. Only downfall of Road Angel is it is not bright enough especially on a sunny day. I emailled them about it, but no reply although they are quick enough to phone you when the subscription is coming up for renewal.

Tim in Scotland wrote:

There is a 10% margin of error permitted on uncalibrated speedos so at 70 speed limit you can be +/- 7 mph out. You can get an indication of errors using a phone app route guidance program online for your phone - one that shows your speed and you can compare the readings. Don’t use the in car nav system as it is an average not an instant reading

Thanks for all the info, will get the app, will be interesting.

Is yours a RRS1 or 2 & what size 22’s are you running.? But as everyone says speedos are always set under the actual speed you are doing,I’m running 295/40r22’s the same as they fit on the svr’s with 22’s & mine is more accurate to the Satnav than it was with the 275’s. Andy.
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Sorry Tyrefitter, it’s a my16 hse 306, still running original continentals cross contact lx 275R40 x22

As far as I'm aware, the accuracy of speedometers is governed by EU directive and must not under-read, and can over-read by up to 10%. I use a sat nav, I have a Garmin, the wife used a TomTom. Give it a try and compare it to your speedo so you know where you stand..

Speedometer 55 Pro is a good app for checking your speed. It also has a feature which reverses the figures and gives you a speed head up display on the windscreen if you put your phone on the dashboard.


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