Thread: Speedo vs GPS

Originally Posted by Nicky

GPS measures distance between points, speedo measures distance travelled over the road, path or track, up and down hills etc. GPS distance measure will always be less than speedo, depending on the vertical and horizontal deviations of the path.

There's also a lag as satellites are acquired, distance travelled during that time isn't recorded.

Speaking to a couple of service advisors, they reckon speedo is out but the odometer is correct. I can believe that, my Commodores were always 2 kmh fast and the VW is 5%. Connecting an elm327 and the speed from software reading the diagnostic port is much closer to gps derived speed.

GPS measures time and distance between points or whenever it refreshes. The total distance shown is the sum of all the distance intervals. It is not the distance between start and finish. As for slope distance versus corrected to be flat I believe that the gps derived distance is the slope distance. Basically they measure lat and long and height. I havent actually tested the distance as to slope v flat, something I should do if even just for ****s and giggles.
Simmo

Originally Posted by DeanoH

I've driven three Mazda Bravo/BT-50's over longish distances and all had a speedo inaccuracy of 8-10 %.

Deano

That's because around -8% is what the manufacturers build in so that the speedos do not over read and therefore contravene ADR 18/03.

Originally Posted by Hugh Jars

^^ and will only give an accurate speed when travelling in a straight line on level ground

& not in tunnels

Originally Posted by Narangga

That's because around -8% is what the manufacturers build in so that the speedos do not over read and therefore contravene ADR 18/03.

I've heard this excuse before and quite frankly don't believe it. My 2004 Disco 2a's speedo was spot on. My 1966 SIIA speedo was more accurate than this. I don't believe that the auto manafacturers are incapable of designing/mass producing a reasonably accurate speedo. To blame gross inaccuracy on ADR compliance is IMO a Furphy.

Deano

My 2008/2009 D-Max came std with 245/70.16 ... Compared against the GPS in a straight line run, the speedo says 100kmhr but the GPS says 93/94kmhr ... I changed the tyres to 265/70.16 and now the GPS says 99/100kmhr when the speedo says 100kmhr ....

... Then I notice sometime later that most of the rest of the world markets get the D-Max with 265/70.16 std ... But in Oz it's the 245's ... I believe ADR's force manufacturers to have speedo's that over-read

Mum and Dads MUX is out by around 6-7% ... Wife's Subaru Impreza at around 118/119 on the speedo shows 109 on the GPS... My 2004 D2a at 100 on the speedo shows 106 on the GPS and 106 on the Nanocom with the tyre coefficient set at 1.06 ... Tyres ARE 6% oversize. Older cars seem to be accurate but newer vehicles ALL read more than actual .... Dad drove the D-Max for 5 years, and when he handed it over to me, the trip computer (that had never been reset in that 5yrs) said 7.4L/100Km ... With the 265's fitted I can't get anywhere near 7.4 ... after I figured out the speedo error issue I now understand why he was getting 7.4

I put more faith in the GPS than the speedo ... Apparently we're not fit to drive a vehicle at 100

The speedo must never read less than the actual speed by law. So, if the speedo says 100 km/h, you're probably doing about 96.
The odometer should be pretty accurate, but a GPS should always be spot on to within maybe 0.5 metres.

https://www.racq.com.au/cars-and-dri...peedo-accuracy

Sent from my GT-P5210 using AULRO mobile app

Mine (running standard size tyres and rims) is out by 6Km at 100.

If i sit the speedo on exactly 106 ...by GPS I am doing 100