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

A while ago I compared three different GPS units against each other in a car...all three were within reading error of each other. Since then I have checked several speedometers against GPS and found that my Integrale over read by about 6%, the Fiesta about the same, the M3W was a full 10% over till I re-calibrated it to about 1% over and the two Hondas I've had were both bang on. I'm very surprised at any vehicle speedometers under reading compared to GPS: out in the open on a straight road and steady speed I would be extremely surprised if any GPS isn't very accurate indeed and that's without going to the more exotic and expensive units that are used for official testing of such things.

Well Belle under reads by c 10%. This is not just the parallax of the speedo in front of the driver. The revs are a useful guide 2k is 20,30,40 & 68 in respectively 2ns ,3rd 4th & 6th. As I can see the speedo under the tonneau the GPS is very useful & matches those roadside signs that light up in a 30 limit.

Are you a chancer?
Do you push the “+10%+2” guide to the limit?
The checks of speedo accuracy versus GPS give an idea of how much leeway you have around the speed limit you are in. Do you then add in this to your living on the edge style?
I only push the limits occasionally but try to keep a couple of mph below the on the edge speed. Watching your speed to that ultimate degree can be distracting and create fatigue.
By using the speedo with an estimate of under reading you have that margin to help relax.
As John says, another guide is the flashing roadside speed sign. You then have 3 speeds to confuse you!

When I collected Belle from Bell & Covill I went round the M25 through the Dartford tunnel to the A12. Speed camera hell frankly. I couldn't understand why everyone was driving 65 mph. When the traffic dropped I went at an indicated 70, with my head in the passenger area to avoid parallax & seemed to be overtaking everyone so I slowed down.

I then tested the speedo vs my Garmin & realised I was lucky to have a license still. MMC said the car was legal despite reading under by 10%

If I take the iPhone, mounted high on the dash with good sky view on all cars as the common medium.

Aero S2 - 70mph GPS shows 84 on the speedo.
Aero Coupe - 70mph GPS shows 79 on the speedo. (from memory as she has not been out this year...)
MB ML63S - 70mph GPS shows 69 on the speedo.
Mini R34 - 70mph GPS shows 72 on the speedo.

The last time I got a speeding ticket was just like you JV6. Visited Cardiff to see a customer. Totally clear on a late drive home and failed to realise that entire M4 section was average speed 60. Thought people were just taking their time as the traffic was light. Got a 68mph ticket. Thankfully a long time back now.

No surprises there, The Plus 8 Speedometer is hard enough to read anyway. At 70 on the GPS it shows about 80 on the speedometer.
The Tesla seems to be very accurate and it is interesting when passing the illuminated speed measurement signs in villages: some agree 100% with the speedo and GPS, others show me doing 32 when the Speedo and GPS shows 30.

Ah, after reading this thread I've realised I transposed the numbers......

So..........



I certainly don't push my luck, because we're under the gantries a lot around here, but I also want to optimise journey times.

That's more like it Dave.
Nick

Very interesting these comparisions but I believe one must also take into consideration the tyre size as well. If one changes tyres and cannot get exact replacement as delivered new or ops for say a slightly wider say 165/70 instead of 155/80, the outer circumference is increased or decreased. If decreased the speedo will show a greater number and vice versa if increased. Just my humble input;)

Across my 3 Mogs the speedos read an array of data: +8 reads approx 6mph above actual speed, the 4/4 reads when it wants too and that is not often, the M3W reads something which I think is altitude it certainly isn't speed.