Don't waste your time trying to use a GPS as a defence mechanism if defending a speeding fine, you will be wasting your time.
Glad to hear that the GPS evidence won't be accepted as proof. I only hope that it works both ways. Cause I never want a cop to look at the maximum attained speed recorded by my GPS. Chances are the log will show much higher numbers than he/she clocked me.
Changes to wheel & tyer size will make a difference
Mechanical Speedo's can wear
Electronic Speedos can also be out (look at all our clocks)
You get caught over the limit no excuses accepted
In some States the police & radar give no tolerance
Even GPS's can be out (triangulation of the Satellite's relative to the car and terain
In Australia by the ADRs vehicle speedometers cannot read lower than the actual speed. Hence, generally they will read faster than the speed you are travelling, this can be 4-5% or more. Your GPS will generally be more accurate than any speedometer in a vehicle when travelling at a constant speed. As tyres wear the diameter changes so brand new tyres will read a different speed to worn tyres, the vehicle cannot compensate for this, even though the difference between worn and new tyres is a very small amount.
Interesting to know the ADR's say the Speedo cant read lower err on the side of caution
Did a quick of my spedo today (out of interest) and sure enough at 100kph there was about a 4 Kph difference. But than I have changed my tyer's from a Bridgestone to a Coopers ATR (slight size variation)
Not too sure what the manufactures actually do but with the two models available there is a 25mm (1 inch) variation in wheel diameter
To be accurate, the ADRs allow tolerance of minus 1/2 percent to plus 10%.
Invariably the manufacturers aim to be in the the zero to +5% range, although larger errors are not uncommon. The reason for this is the need to cover a range of tyre sizes with a limited number of speedo gear drives. The same effect can be induced by fitting tyres if a different circumference. A lot of young guys don't realise that a 205 x 45 tyre is not the same diameter as as a 215 x 45 of the same rim size - the 45 being a percentage of the width.
I have noticed a difference of about 6 km/h lower on my Nuvi 660 at 100 on car speedo. With all the speed cameras around I wouldn't trust your GPS speed as being correct. It might be costly.
Previously I had a PDA with navigation and it was exactly the same as the car speedo. It used Co-Pilot Live with an external GPS receiver.
On marine GPS units they calculate speed over ground which is always slightly different to speed through water. Maybe this is why there is a difference.
Another thing to consider is that GPS's calculate your position every second or so so that it calculates your speed between these points in a straight line, roads tend to curve, so you are actually driving slightly further (so slightly faster) than the GPS thinks you are and the tighter the bends the greater the discrepency, but as billynibbles mentions with the hills this is likely to be a small effect, but all of these small effects add up.
With the accuracy of GPS units down to 3 metres these days and speed of processors etc, don't think a few bends would affect the output readings.
Would much rather believe the GPS over vehicle speedo's these days.
There is no excuse for speedo errors if the original tyres and pressures are used. Problem arises when manufacturers use different profile tyres as on model variants and do not re-program digital Speedo's. Had a Honda 2010 and Suzuki 2012, both sports models with low profile three and both were 8 kph honda and 10kph suzuki out, both above actual speed.
Legislation in most countries requires the manufacturer to guarantee speedometer readings to an accuracy of only 10%.
Therefore it is obvious that a GPS reading....generally highly accurate....will be far better than a mechanical reading in a car speedometer.
It could be more....it could be less.
Go with the GPS reading.....it could save you a ticket.
It is really normal that GPS's speed is slower than the Car's speedo. Manufacturer will increase speedo for safety issue, by contrast the GPS's speed is calculated by satellite based on the vehicle movement. Thus, trust GPS's speed.
I experience the same thing with my Mazda. It is always about 5km more than the GPS. I think this is a good thing as it would mean I will never speed as long as I keep to the speed limit as indicated on my speedo.