MPH on Satnav

londonTiger

forgotmyname wrote: »

Sat Navs are not 100% accurate. Experiment and see how closely it can plot your position.

Sometimes its several feet out, How can something thats only accurate to several meters be 100% reliable for your speed?

Stick to what your speedo says.

when you have your cruise control on and keep a constant speed and test it again your satnav over a long distance. the "several feet out" becomes negligible difference over substantial distance. Just make sure you're going through a long straight flat road. preferable a motorway or carriageway at night when you dont have obstructions.

If yuo're into aftermarket modding you can buy a aftermarket speedo corrector and fit it yourself to get an accurate reading. This is commonly done when cars are kitted with larger wheels that the ECU doesn't recognise.

Hintza

forgotmyname wrote: »

Stick to what your speedo says.

That is about an 8%-12% differential (higher at lower speeds) for me.


At the end of the day you have to do what you are comfortable with.

daveyjp

My sat nav and speedo read exactly the same.

longtimelurkersam

daveyjp wrote: »

My sat nav and speedo read exactly the same.

Mine do in the current vehicles, but I prefer to look at the sat nav speed on the windscreen as it is almost always in my line of sight and I have been doing it for so long now that looking down at the speedo seems dangerous. Obviously I know it isnt ;-)

My old Transit overread by a HUGE amount. I just ignored the speedo and always had the sat nav on.

I also cruise control on satnav speed through SPECS on the motorway.

mattyprice4004

scooby75 wrote: »

Your satnav probably is more realistic of your speed. Speedometers as I understand cannot read less than the speed actually achieved, so in-car speedometers are manufactured to read a couple of mph excess.

But as you point out, your satnav varies in the speed so is probably less realiable. At the end of the day, I would trust my speedometer than may satnav. That's its designed purpose and if anything went wrong (e.g, caught speeding when I wasn't as a not necessarily realistic example), the speedometer in the car would be the reference used in my defence.

Sat nav will always be more accurate than a car speedo.

marlot

mattyprice4004 wrote: »

Sat nav will always be more accurate than a car speedo.

No it won't.

It will only be more accurate when doing a constant speed on a level straight road.

It will be less accurate when the car is changing speed, going up/down hills or going on a twisty road.

I used my satnav to work out just what my speedo showed when I was doing a genuine 30/50/70, and then used the speedo.

redux

Actually a hill will have not have a lot of influence on the sat nav indicated speed.

It should be measuring an actual 3 dimensional velocity, not a horizontal projection. If it was in a fighter aircraft or missile climbing near vertically the indication should not be down somewhere near zero, but the actual speed and direction it's headed in the interval since the last calculation.

It's true though that depending on satellite positions GPS can be less accurate in the vertical dimension, but I don't think that's going to affect a car.

But even if someone has programmed a road sat nav differently, the error would not be significant

On a 1 in 6 slope the error would be 1.5%. On a maximum slope on a UK motorway, 3% or 2 degrees, the error would be less than 0.1%. If that seems hard to believe, look up some sines and cosines

But as I say, I don't think they will be programmed like that.

People mention that there is a position uncertainty of a few metres. But that doesn't change from instant to instant but over longer periods, so calculation of the speed between two points will tend to remove that common mode and make the speed more reliably accurate than the actual position.

A sat nav will certainly be more accurate than a car instrument.

steve-L

Which is more accurate, a micrometer or 100m tape?

If you want to measure in microns then a micrometer, if you want to measure a field circumference a 100m tape!

The satnav only measures change in location relative to the last position.

Actually a hill will have not have a lot of influence on the sat nav indicated speed.

The GPS for car or marine use assumes a flat plane. (As do the ones for use in light aircraft) ... the reason being the owners of the GPS system would rather they were less useful for sticking in missiles as ell as why bother .....

A sat nav will certainly be more accurate than a car instrument.

Either will only be as accurate as calibrated.

one is dependent upon tyre circumference, one not. If you calibrate the one for the tyre then within a few microns of wear/temperature and pressure it is capable of accuracy less than 0.1mph .. it is most certainly easily accurate to 1mph....

The SatNav is position fix dependent. If you drive round a long circle it will cut it in straight lines.
If you drove around the M25 at in say exactly 10 hours clockwise then anticlockwise the SatNav would not even reliably indicate the difference in speed, however your car speedometer would,

redux

steve-L wrote: »

The GPS for car or marine use assumes a flat plane. (As do the ones for use in light aircraft) ... the reason being the owners of the GPS system would rather they were less useful for sticking in missiles as ell as why bother .....



Either will only be as accurate as calibrated.

one is dependent upon tyre circumference, one not. If you calibrate the one for the tyre then within a few microns of wear/temperature and pressure it is capable of accuracy less than 0.1mph .. it is most certainly easily accurate to 1mph....

Even if that's right, then as I said the error due to the horizontal projection distance is trivial almost all of the time.

The cosine of 2.5 degrees is 0.9990, which means on the steepest UK motorway the effect is about 0.1%, or about 0.07 mph at 70 mph

Tyre tread depth varies from 8 mm new to 1.6 mm when worn to the legal limit. That 6.4 mm variation is approximately 2% of the radius of most wheel and tyre combinations, for example 195 65 15 has 317 mm radius or 205 60 15 has 314 mm.

So tyre wear between new and replacement accounts for approx 1.4 mph variation at 70 mph. It is certainly not accurate to anything remotely approaching 0.1 mph

And the speedometer instrument error is often more than that, such as the 2 to 5 mph in the original post, and not necessarily a linear proportion of speed.

Tobster86

GPS will be more accurate at constant speed, but will experience a delay when changing speed. Guages must not under read and my over read by up to 10% (vehicle consruction & use regulations from memory).

Both present equally far less safety critical information than what you get from looking at the road & concentrating on possible hazards, despite what the government thinks.