Speedo Calibration
 

Hi, yesterday I passed 2 speed signs which tell you what speed you are doing, both showed, at 30mph, that I was doing 2mph less than my speedo was showing (XC60, 2020). I know that there are manufacturers tolerances for vehicles, and these road signs, and it's better that your car is doing a little less than what your speedo is showing. Also there may be differences when swapping wheels, eg. summer to winter. I'd like to know how or where I can get my speedo calibrated and the cost. It would be useful to know what speed I am actually doing, at different speeds. Does anyone have any recommendations?

You need to go to a garage that does tachograph calibration.
https://www.gov.uk/find-approved-tachograph-centre-atc

I've never heard of anyone other than the Police having a calibrated speedo in a car, or even one that can be calibrated. I work as an HGV driver and our vehicles are fitted with tachographs which have to be calibrated every two years, but even then they are not totally accurate, from various on the road reckoning such as side by side running and timing over measured distances (yes the roads are very quiet in the small hours of the night) we know even a newly calibrated tacho/speedo can be 1.5 to 2% out, and worn/new tyres only add to the inaccuracy. Comparing speeds displayed on those roadside signs with the speed displayed on the dash, allowing for the known inaccuracy of the tacho I've also observed that those signs are often not very accurate either.

In the UK your speedometer can legally over read by up to 10%, but not under read. Most cars are a few MPH fast, my fiat is exactly 10% over, so when you are doing 70mph on GPS it says 77mph on dial.

All volvos I have had tend to be between 5-7% over reading. I normally check against my GPS speedmeter app on phone and also dash cam GPS.

Changing tyre size will affect it as well, and tyre pressure (depending on load).

For example common V70 size is 225/50R17 with diameter 658mm, my V70 has 245/40R18 which is 653mm, about 0.8% smaller which will result in the speedometer reading 0.8% faster.

I would download the Speedometer app and check against that.

Try Instrument Repair Services of Nottingham

They used to have contracts to calibrate Police speedos so may be able to help.

A calibrated police speedo has the accuracy checked and is done to the tyre size on the vehicle and also has the markings on the face with each mph rather than every 5 mph.

As your car is new I would assume that any form of recalibration would invalidate the warranty.

Thanks. There are several "Speedometer" apps. in Appstore & they all seem to rely on GPS, are there one or 2 you would specifically recommend?
It also begs the question of how accurate GPS is in showing true speed, but if it is reliable then one could use an app in different cars etc.
I wouldn't want to get my speedo re-calibrated as I agree it could invalidate the warranty.

Thats about right - as stated ^^^^ I'd expect the speedo to show a slightly higher speed than the actual speed being travelled. A 2mph difference is minimal IMHO - remember that most speed cameras (and I'm guessing that this is your actual concern) allow 5-10% (depending upon the county/Police force) anyway.

An old trick in the haulage game was to send a truck for tacho recalibrating and speed limiter setting on a set of worn out third re-cut tyres, then fit a good set of tyres, ie taller, to get a few more mph out of it.

Don't rely on that around Gloucestershire and Avon and Somerset, 1mph can get you a ticket on the M5 :(
Used to work for a company that kept a special set of wheels with worn out tyres in the workshop just for calibration days :D
The problem with speed measurements is they are derived from time and distance, electronic clocks are now usually accurate to seconds within a year so virtually spot on, the distance measurement is where error creeps in, as noted already a worn tyre will be a noticeable percentage smaller than a new one so the speedo will over read by that amount. GPS speeds depend on accurate distance measurements from the GPS device, using a Running app on our phones that uses GPS both my wife and I have found that on occasions the app will declare a different distance for the same route, sometimes 0.5k on a 10k run, this is probably because the app only monitors position at set intervals and then calculates the distance as a straight line between those points, if you've gone round a corner the GPS app takes a short cut across the grass. I also found that when I stopped to talk with a friend last week that the app auto paused but then restarted and paused several times as we chatted, the map of my route showed me meandering to and fro over about 20m while I was actually stood still. Generally GPS is OK, just not 100% accurate 100% of the time.