I was out in my 2004 Z4 today, using my BMW badged, Garmin screen mounted, Sat Nav, when I noticed the difference in the cars speed being displayed on the Sat Nav compared to my speedo. The difference was roughly 4 to 5 mph, with the Sat Nav showing the lower speed. I assume that the Sat Nav is the more accurate of the two, but has anybody else noticed this sort of difference??
SAT Nav versus Speedo
- Thread starter olsonn
- Start date
Satnav will be the most accurate, in car speedo will almost always be slower
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I think the speedo has to be within 10% when manufactured but always over estimates. So in theory a 100,000 car may only have done 90,000.
Rob_benton said:
I'm not sure the odometer will misread, but the speedometer has a built in error. Be a little careful relying on the Satnav as they tend to have an update "refresh" time and on bends, hills etc they will misread.
Its in the manufacturers benefit for the speedo/odo's to read high, think about it, if it reads high then your car feels faster than it really is, your servicing comes around quicker & your perceived MPG is better.
They rarely read low, imagine if you got busted for speeding when you swore blind you were only doing say 70mph as indicated on your speedo, but were actually doing 77mph
Trust the sat nav speed as long as you are on the straight & level.
They rarely read low, imagine if you got busted for speeding when you swore blind you were only doing say 70mph as indicated on your speedo, but were actually doing 77mph
Trust the sat nav speed as long as you are on the straight & level.
Just realised my comment was arse about face. The car speedo should always read faster. So in other words you aren't going quite as fast as you think you are. I think law states your speedo cannot read slower than actual speed.
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Bing said:
The car speedo will over read, the Satnav will normally show slower. Interestingly, I get a 1mph difference between my RoadAngel Gem and the speed reported by the BMW Nav via the Intravee.
MotorcyclesFish
Member
I'd imagine that the way they do it is with a true speed value derived from the speed sensor for the specific gearing of the car (transmission, LSD, rolling radius) used for satnav calcs and odo reading, etc. Then it's multiplied by 1.1 or thereabouts and *that* fudged value fed to the speedo stepper motor.
Otherwise you'd have inaccurate satnav timings and a class action lawsuit based upon fraudulently accelerated service schedules.
Otherwise you'd have inaccurate satnav timings and a class action lawsuit based upon fraudulently accelerated service schedules.
I would say they always do it a bit higher for legal reasons. My actual distance is not 10% more than when I check a journey on eg, Google Maps.
If it was then I would be overclaiming fraudulently from the taxman and my employers, for business mileage.
The only time I have ever had a corresponding speedo/GPS reading was in the US, I had a RAV 4 hire car with very little tread on the tyres.
Maybe the speedos have to be more accurate over there? or maybe the non-premium hire company fitted smaller tyres as they were cheaper( there was 50k on the car)
If it was then I would be overclaiming fraudulently from the taxman and my employers, for business mileage.
The only time I have ever had a corresponding speedo/GPS reading was in the US, I had a RAV 4 hire car with very little tread on the tyres.
Maybe the speedos have to be more accurate over there? or maybe the non-premium hire company fitted smaller tyres as they were cheaper( there was 50k on the car)
sammysquid
Member
The law states that the speedo must not under-read, but given there is going to be error in readings that can't be removed the manufacturers always set the speedo to read slightly faster therefore even if there is a variation it would not under-read.
The margain of error has to be as mentioned before <10%.
People often state that they can be allowed to do the speed limit plus 10% as speedos are 10% out but this isn't true.
The margain of error has to be as mentioned before <10%.
People often state that they can be allowed to do the speed limit plus 10% as speedos are 10% out but this isn't true.
The speed shown on the speedo doesn't even match the speed taken from the OBD port via a data logger.
Mine, with the larger 265 rears, shows a 3-4% higher reading on the speedo than the GPS or data logger - e.g. I have my motorway cruise control setting at 88mph on the speedo and my satnav shows 85mph.
Mine, with the larger 265 rears, shows a 3-4% higher reading on the speedo than the GPS or data logger - e.g. I have my motorway cruise control setting at 88mph on the speedo and my satnav shows 85mph.