difference between sat nav & speedo

[biggs682]

on this mornings trip i noticed that the difference in mph between cars speedo reading and sat navs was only 1 mph . i expected a bigger difference.

what difference do you get

car is an early Picasso , sat nav is a basic Mio

 

[Drago]

It depends but. A speedo measures your ground speed a surface, which will undulate up and down.

A test nav measures you speed between two fixed points in space (it may use half a dozen satellites, but each individual measurement will only between 2 birds).

So, a from A to B measured by GPS may be 1 mile exactly. However, because the distance along the surface may be 1.05 miles, hence differing readings.

As a general rule Mororwats etc should give a fairly accurate GPS reading of your velocity along the road surface, but in the Peak District it'll well out.

 

[swansonj]

Current car speedo reads 3-4% over true speed at 30-40 mph rising to 6-7% at 70 mph. I used to test car speedos by driving at constant speed between kilometre markers on the motorway before Satnavs and got up to 10% fast on older cars. Car speedos are only required to be 10% accurate but are of a technology that can easily be manufactured to a couple of percent, so it is widely believed car manufacturers specify speedos that read a few percent over, perfectly legally, to make owners think their car's performance is better than it is. That was probably quite a good thing even though pandering to not very good instincts.

 

[fatblokish]

It depends but. A speedo measures your ground speed a surface, which will undulate up and down.

A test nav measures you speed between two fixed points in space (it may use half a dozen satellites, but each individual measurement will only between 2 birds).

So, a from A to B measured by GPS may be 1 mile exactly. However, because the distance along the surface may be 1.05 miles, hence differing readings.

As a general rule Mororwats etc should give a fairly accurate GPS reading of your velocity along the road surface, but in the Peak District it'll well out.

?

 

[Deleted member 26715]

The drive normally comes off the gearbox final drive, so 1 turn of the driveshaft with the tyre correctly inflated with full tread will take the car a set distance, so the speedo is effectively counting the number of times the shaft goes round & then calculates the speed. However if your tyre pressure is low or you have less than full tread on your tyres these calculations get skewed. I have also been told that a Sat Nav will only tell you an accurate speed on a flat road, as if you are either going up or down a hill it cannot compensate for that, how true this is I'm unsure.

Alan...

 

[avsd]

My GPS is usually within a few mpg of the cars speedo

 

[Night Train]

When my speedo says 70 my satnav says 65.
When my speedo says 30 my satnav says 28.

My satnav has a slower reaction to speed changes then my speedo does.

 

[DRHysted]

I can actually obtain the cars computer speed reading, which is slightly lower that the dial. When I have done this, the cars computer reading generally matches the speed shown by the sat nav.

 

[threebikesmcginty]

The other day when the Porsche said 175mph on the M40 the satnav was only on 165mph, I had to phone a mate to ask if his was the same, dropped my coffee in the process and then the burger and fries on my lap went everywhere, to top it off some old duffer in front of me braked and nearly caused an accident.

 

[swede54]

A bit of school geometry makes nonsense of the GPS gets the speed wrong on hills. Most GPS units update once a second, or every 30 yards at 60mph, but the satellites are 12000 miles away so whether or not you are going on the flat is insignificant.

 

[ColinJ]

If you were going at a constant velocity then you should get a very accurate reading from your SatNav. I assume that the devices sample your position at regular intervals and calculate the speed by assuming constant velocity between two points. The less constant your speed, the less straight the road and the longer the sample period, the less accurate the SatNav would be. (Having said that, surely they sample frequently enough that they are pretty accurate?)

 

[oldfatfool]

By law a car speedo can not under read. I have found Fords to have the most accurate speedos compared to sat nav whilst both the citroen and the skoda will be over by around 10% compared to sat nav.

 

[Smurfy]

According to Wikipedia the maximum allowable speedometer error is -0% of the actual speed, and +(10%+6.25mph) of the actual speed.

 

[Alien8]

GPS receivers can determine instantaneous velocity by measuring range rate (Doppler) from the recieved signals. In reality I would expect the software to produce a smoothed position and velocity solution by combining all the data in a Kalman filter with a suitable dynamic model. This can be why you see a lag between your cycle computer speed and GPS derived speed. In the same way that the geometry of the satellites tends to give a more accurate horizontal than vertical accuracy the horizontal velocity will be more acurate than the vertical velocity. If a GPS receiver determines speed by simply differencing position fixes then it has poor software.

 

[Smurfy]

GPS receivers can determine instantaneous velocity by measuring range rate (Doppler) from the recieved signals. In reality I would expect the software to produce a smoothed position and velocity solution by combining all the data in a Kalman filter with a suitable dynamic model. This can be why you see a lag between your cycle computer speed and GPS derived speed. In the same way that the geometry of the satellites tends to give a more accurate horizontal than vertical accuracy the horizontal velocity will be more acurate than the vertical velocity. If a GPS receiver determines speed by simply differencing position fixes then it has poor software.

Good stuff, but how do I know whether my mobile phone GPS is using 'differencing position fixes'?