Talk Morgan Forums Morgan Sports Cars Morgan Aero 8, 2012< Plus 8, SuperSports, Coupe & Aeromax Over enthusiastic speedo

Actually, now I remember having to do this with the speedo in my Beach Buggy

I too use the satnav as a more accurate indicator of speed than the speedo but I have often wondered if it really is a true indicator of speed.

Is there any knowledgeable person out there that can confirm the accuracy of satnav indicated speeds?

Hi
There I have used my sat nav in my cars and the only one that reads the same speed on speedo and sat nav is my Plus 8, the Volvo V70 reads 2mph lower on speedo than sat nav, the Nissan Terrano van 4x4 reads 4mph lower on speedo than sat nav.
I know the 4x4 does read lower than actual because of tyre sizes, the speedo on plus eight is near perfect in calibration no error.So I would say Sat Nav is speed over ground and is more accurate than most production cars which can have up to 10% error below, must not be above.
regards
Phil

If you are doing a steady speed on a straight road, the sat-nav should be spot on. However they don't adjust quickly enough to give an accurate instaneous reading during changes in speed and/or direction. Tall buildings don't help as the sat-nav can miss some satellite bleeps and then it just estimates your position until it gets another reading.
So they are perfect for calibrating your speedo on a long straight (eg a motorway), but not, in my view, suitable as an everyday speedo.
I can't say I speak with any real authority on the subject, but this how I believe it works.

Fjemog

A straight, horizontal road...
Assuming a fixed speed on horizontal road, a change in altitude would increase the speed reading (think Pythagoras and triangles).

Cheers,

I'm doing this from memory having looked at it some time ago and with Wiki being switched off.

The accuracy of a SatNav will be affected by a number of factors one of them being the number of satellites its working with, the more satellites the more accurate the location.

For speed accuracy is improved with the frequency of communication between the SatNav and the satellites. This is the bit the allows the military to send missiles around the world to with a few feet of target. Just looked it up and the accuracy of a domestic satnav is +/- 10 metres whereas the military is less than 1 metre.

Translating that into road speed accuracy will be an issue.

On that note I went to see a client today whose wife works in a scammervan department.

He tells me that her department is set to becoming self funding from the revenue received through driver training courses.

You'll be surprised to hear that in this particular police area the number of people attending driver training has risen significantly and is being looked at as the model for other forces.

My Garmin always gives me the altitude - I think the satellite info is 3-D (it must be, surely?), so it should calculate the true speed whatever the gradient. Anyway, even if that is not the case, you would need a fairly steep hill to make an appreciable difference. My trig suggests a 10% grade (1 in 10) would give an error of 0.5% - most long straight roads are a lot flatter than this.

Cheers

Fjemog

Interesting discussion on the accuracy of GPS devices.

For info when I did my speedo, I used two devices, a traditional satnav and the Avertinoo "danger spot" device installed on my phone. They both gave the same speed though occasionally differed by 1km/h. Of course that's not to say that they are both right so I have done a bit of additional testing, or what some might call Russian Roulette.

French speed cameras are set with very little room for error. So for example if you are going at a real 97km/h past a camera in a 90km/h zone, you will be flashed. Odds are you will then receive a ticket (assuming they can trace you) for doing 91km/h in a 90km/h zone.

With my newly calibrated speedo I have driven at an indicated 95km/h through a 90km/h camera, and at an indicated 140km/h past a 130km/h camera. In neither case have I been flashed!

Cheers

Unless it is one of the new changes in the MOT, a speedometer is not actually a requirement.

It's part of the new MOT rules. Hs to have a speedo fitted and be working correctly.