Thread: Speedo vs GPS

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.

GPS derived speed is correct, virtually cannot have error if travelling at steady speed. It depends on the refresh rate when you are changing speed. GPS derived distances I believe will be correct as well.

Vehicle odometers (distance) are quite unreliable in my experience usually with a constant scale factor. Vehicle speedo's likewise. Most are configured to run at about 6% error, ie 100km indicated = 94km real. This error is not neccessarily proportional over the speed range though. Every time I have checked my GPS speedo against one of the overhead speed radars they are always showing the same as the radar. This has been with maybe 6 different GPS units and with many of the overhead radars.

So in short the GPS odometer will be nearly exactly correct, the vehicle odometer is another storey though.

Cheers, Simmo

Originally Posted by BobD

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.

Yep 285/60 R18's are pretty close to accurate, standard sizes like any vehicle are always under.

Originally Posted by BobD

I've compared mine against the distance pegs on the side of the road and it was spot on with standard tyres and now about 5% low with larger tyres. My speed was high with standard tyres and now spot on with the larger tyres.

I've had the same experience as Bob, except mine is a 1-2% margin because I have the 265/60/R18 which is only 1-2% different from the 255/55/R19

Scott

Originally Posted by simmo1

GPS derived speed is correct, virtually cannot have error if travelling at steady speed. It depends on the refresh rate when you are changing speed. GPS derived distances I believe will be correct as well.

Vehicle odometers (distance) are quite unreliable in my experience usually with a constant scale factor. Vehicle speedo's likewise. Most are configured to run at about 6% error, ie 100km indicated = 94km real. This error is not neccessarily proportional over the speed range though. Every time I have checked my GPS speedo against one of the overhead speed radars they are always showing the same as the radar. This has been with maybe 6 different GPS units and with many of the overhead radars.

So in short the GPS odometer will be nearly exactly correct, the vehicle odometer is another storey though.

Cheers, Simmo

Just be careful that from memory the odometer and speedo do not use the same calibrations. For instance the odometer may run true but the speedo is set in the CCF to run with a % error.

Originally Posted by cjc_td5

Just be careful that from memory the odometer and speedo do not use the same calibrations. For instance the odometer may run true but the speedo is set in the CCF to run with a % error.

Absolutely! If you check at what displayed speed the vehicle will drop out of off-road height you will probably find it's around 53 kph, being actual 50 kph but modified for display by 2 kph + 3.5% because it has calculated the proper speed using the correct distance.

My notebook's GPS displayed speed varies near trees, jumping up and down by several kph.

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

My current Bravo came with 185R14's and the speedo reads 109 Km/hr when the GPS tells me I'm doing 100 Km/Hr. 'Upgrading' the wheels and tyres to 205/75 15's has resulted in the speedo reading 100 Km/Hr when doing 99 Km/Hr which is good enough for me. The resultant change in gearing (manual transmission) makes the vehicle much more driver friendly as a day to day, non loaded, drive. The handling and comfort is also markedly improved.

My cynical view is that the manufacturers engineer this deliberately to give the impression that the vehicle(s) give much better fuel consumption than they really do.

Why do you think the hire car companys love BT-50's. I had a major blue with a hire car company in Mildura several years ago when I deducted 10% from the distance travelled. They pretended to be shocked and horrified but the bastards knew all along and caved when push came to shove. Not bad when they're creaming an extra 10% on the odometer reading.


Deano

I wonder what would happen with a warranty issue if it was a couple of hundred kms out of warranty ?

Martin