Talk Morgan Forums Community The Malvern Arms Update on GPS Speedo test

Well, I've tested my fleet, and here are the results.

2012 Golf, GPS indicated 70, Vehicle speedo shows 65mph

2016 Polo, GPS 70, Vehicle speedo 68.5

2005 Roadster, GPS 70, Vehicle speedo 68.5

2016 Plus 4, GPS 70, Vehicle speedo 69

Interesting result.

That is interesting. What gps did you use? I’m a sad as you as I’ve done similar things with all my cars for as long as gps has been available. (20 years?). In every case the car speedo has been higher than the gps readings. For example my Audi Q5 indicates 30 at gps 28mph.

You cannot rely on the GPS readings being correct for various reasons i.e.number of Satellites being picked up, their position, signal lag and quality of the receiver. DON'T get a ticket relying on your GPS it wont get you off in court.

It's the one from Demon Tweeks. All its done really is to confirm how accurate the car speedos are, except the Golf.

That's interesting because manufacturers usually build in a factor of around 6% of over read into the speedometer i.e. your speedo reads 70 and you are really doing 66 - 67. This protects them from being sued and also means if you are caught speeding you are speeding, any alteration to the speedometer also null and voids the manufacturer.

I suspect your GPS is telling porkies Dave. There are a number of ways and variations of same on how GPS speed readouts are calculated and displayed. Number of satellites locked to, damping over time, discarding rogue positions, rounding of calculations etc.

When I had a sailing yacht, the Garmin GPS mushroom I upgraded to had regular firmware updates I installed. One allowed me user customisation to adjust speed displayed by altering damping, choosing how many decimal points, sample rate etc. as it's impossible to optimise for all speeds and conditions of use. Choosing a longer sample time gave slight delay on average speed readout but a more constant reading in rough seas. Short sample times confuse as the positon hops about due to vaguries of reception allowing momentary wild speed variations.

I have compared 2 different GPS units to speedos on several cars & bikes & they all read more than the GPS speed. Closest was Chrysler 300C with a 1 mph difference.

"The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph."

https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

Interesting thread for me as I've tested all my cars against Gps and in every case the speedometer in the car has been optimistic. Typically for a gps reading of 70 mph the car says 74 or 75. At 30 on gps cars read 33 or so.
You never can tell with science!
Nick

Checked against GPS our three cars VW polo GTI, Fiesta Titanium tractor and Plus 8, they all read 5-6 mph high at 70 and 3mph high at the 30 point.

I have always used the speedo to go by and have only had 3 speeding fines over the 53 years of driving my last being in 1991, so I'll stick with my method and ignore the semantics.

Keith's advice is the best tbh he's at the coalface.