General Speedo accuracy

[Lloyd71]

A high end system probably would but commercial sat navs don't go into decimal points so you wouldn't notice unless the gradient was insanely steep and went on for a long time.

 

[jrkitching]

It is easy to be sceptical about the abilities of GPS but modern units provide a degree of accuracy undreamt of 20 years ago and in all cases far more accurate than any but the Police calibrated speedometers.

Agreed, though not all are created equal, and the quality of GPS application software is variable. My new smartphone's internal GPS is only accurate to ~ 20m, but if I connect an external GPS receiver, that comes down to about 2-3m. Connecting an external aerial to the GPS receiver improves signal stability.

To those debating the effect of hills on accuracy - remember GPS is a 3D system and gives you an altitude component in addition to Lat/Long, so if the application software is correctly written, hills will not affect accuracy - a properly coded GPS Speedo application should still work properly even if you drive off a cliff.

 

[TrevC]

I checked mine with my new satnav recently, and naturally the speedo over-reads. It doesn't seem a straight percentage error, either - a "true" 30 reads 33 on the speedo, "true" 70 reads 74. These satnav figures back up the speed reading I occasionally see on temporary roadside safety devices that show your speed as you approach them - presumably measured by radar.

 

[loveshandbags]

I checked mine with my new satnav recently, and naturally the speedo over-reads. It doesn't seem a straight percentage error, either - a "true" 30 reads 33 on the speedo, "true" 70 reads 74. These satnav figures back up the speed reading I occasionally see on temporary roadside safety devices that show your speed as you approach them - presumably measured by radar.

I can't testify to this but 'watching' some guy in a A500 on a youtube trying to better the manufacturer's quoted maximum on an autobann in Germany the 'out factor' of the speedo and the GPS speed seems to increase every so slightly exponentially as the speed increases. I wouldn't quote what he was doing and what the GPS was reading but he was out (from memory) around 12mph.

 

[jrkitching]

I can't testify to this but 'watching' some guy in a A500 on a youtube trying to better the manufacturer's quoted maximum on an autobann in Germany the 'out factor' of the speedo and the GPS speed seems to increase every so slightly exponentially as the speed increases. I wouldn't quote what he was doing and what the GPS was reading but he was out (from memory) around 12mph.

Interesting. In the old days of analogue speedos with whirling magnets and springs, they could (and sometimes did) behave in some very erratic ways, but I would have expected today's digital devices to be consistent in their percentage error.

Anything which affects the rolling diameter of the wheels will also affect the accuracy of the speedo, and as manufacturers have to allow for slight variations in tyre diameter, they need to design them to generally over-read, as they need to avoid under-reading even in the worst possible case.


A GPS based device should become more accurate as speed increases, but at typical car driving speeds I wouldn't have thought the effect would be significant.

 

[loveshandbags]

Interesting. In the old days of analogue speedos with whirling magnets and springs, they could (and sometimes did) behave in some very erratic ways, but I would have expected today's digital devices to be consistent in their percentage error.

Anything which affects the rolling diameter of the wheels will also affect the accuracy of the speedo, and as manufacturers have to allow for slight variations in tyre diameter, they need to design them to generally over-read, as they need to avoid under-reading even in the worst possible case.


A GPS based device should become more accurate as speed increases, but at typical car driving speeds I wouldn't have thought the effect would be significant.

JR- I think you knew what was going to happen next.

At the very start of the clip - the first 6 seconds the GPS speed read 225kmph while the speedo reads 240kmph - a difference of 10mph and this was on a standard A500. There is a modified A500 with a bigger turbo and 1.6 engine (a GTech - a guy from the Abarth forum) but I can't say for sure if he managed to get a 12mph difference.

 

[RobinPJ]

That must rank as one of the most boring things I have ever watched.

 

[loveshandbags]

That must rank as one of the most boring things I have ever watched.

PJ - the only 'bit' that was of relevance was the first 6-7 seconds on the GPS and the Speedo readouts to illustrate the difference in readouts (I did mention that on my posting). I'm not too sure what you were expecting to see . The youtube was an unrestricted section of an autobahn in Germany and thankfully it was uneventful.

 

[RobinPJ]

Sorry Michael, that was not intended as a criticism of you for posting it, just an off topic comment on the banality of that sort of video.

 

[loveshandbags]

Sorry Michael, that was not intended as a criticism of you for posting it, just an off topic comment on the banality of that sort of video.

That's no problem PJ. You'll notice from a previous posting that I had resisted posting the clip since I wouldn't condone someone attempting the same stunt. Realise that you were in that 'fired up mode' when a dealer was possibly trying to pull a stroke on someone's deposit.

 

[Most Easterly Pandas]

Almost all cars read +10% on both speedo and tachometer. BMWs being a notable exception.

I've never known it on a tacho (engine speedo tacho I assume your on about?).