Accuracy of Vehicle Speed

Jun 6, 2010
Cumbria, England
TomTom Model(s)
XL IQ Routes Edition (UK & ROI)

There is quite a large discrepancy between the speed of my vehicle as shown on the speedometer and that displayed on my TomTom XL IQ Routes Edition. I realise that the speedometers of vehicles are not precise but does anyone know how accurate the information displayed on the TomTom is?
Your GPS speed is about 0,1MPH away from deadly accurate, your vehicle speedo can be as much as 10% out, trust the GPS and ignore the cars speedo (unless you are in a tunnel) - Mike
I realise that the speedometers of vehicles are not precise but does anyone know how accurate the information displayed on the TomTom is?
Per Mike above, exceptionally so.

If your vehicle speedo is significantly off (apart from "large", you don't say how much difference you're seeing) odds are good that the OD (outside diameter) of your current tires don't match those of the original OEM tires. You'd be amazed at how far off you can get by substituting a badly sized set of rubber.

Just for grins -- what make/model/year vehicle are you driving, and what size tires are mounted to it now?
I am not too sure about the rules pertaining to accuracy in North America of vehicle speedos but in the UK we are allowed to over read by up to 10% but not under read at all, this means:

True speed =70MPH, vehicle speedo can read upto 77MPH and is fine
True speed =70MPH, if vehicle speedo reads less than this (69 or lower) then its illegal

Strange point about the tyres - I changed to a non standard wheel/ tyre combination on my Ovlov and the speedo is more accurate than it was with the original wheels/ tyres - Tollerance stacking up on the speedo drive etc makes the non standard fit better in my case for speedo accuracy, I do drive with the GPS speed as my primary speed reference though (except when in tunnels) - Mike
Hi Guys

Firstly, many thanks for all your replies.

At 30 mph the difference is about 2 mph, but at 70 mph my Tom Tom records about 63 mph. This would seem to agree with what mikealder said. My tyres are about half worn and are the original factory fitted ones so that would certainly increase the over reading somewhat.

I did know that speedos weren't that accurate but didn't know that they could read over but not under, although that makes good sense. Glad to hear that I can rely on the Tom Tom to keep me in check!!!

Thanks again.
The speedometer in a car MUST show a higher speed than the real speed. The difference between the indicated speed and the real speed varies between different car models. Thus, the police have a calibrated measurement system, whereas the car owner is relying on a non calibrated system. Furthermore, I don?t think that the car owner can calibrate his/her speedometer at any car test facility. So, from a legal point of view this is strange. However, ensuring that the speedometer of the car ALWAYS indicates a higher speed than the true speed reduces the legal problems. That?s the reason.
To summarize, the car?s speedometer is NEVER indicating the true speed and this is true for all cars in all countries. And in some countries I?ve heard that this fact is also a law.
Someone interested in programming should develop a speedometer (analog or digital) that could replace the map on a TomTom GPS! It shouldn?t be that difficult.

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