Richard H wrote:
Yep see your point.
I get frustrated because the sat nav invariably tells me I'm going slower than the speedometer in the car. That really winds me up.
Well I assume you know the car's speedo will always be wrong due to legislation requiring speedos to never report a speed under actual speed in many jurisdictions. How much it over reports speed depends on the manufacturer's calibration margin allowance to ensure they comply and of course the circumference of the tyres on the wheel used to measure speed. It's often over reports speed by as much as 5-10%.
But then that just brings up another point with your accuracy comments.
While your car's speedo may be inaccurate, it is very consistently so, always over reporting by a fixed percentage. If it does change, it does so only marginally with the evolutionary change in circumference of your tires with wear and inflation levels, and in a step change if you change tires and/or wheels and tires which have a different circumference.
Hence, provided you inflate your tires, you are able to consider your speed reading to be quite precise to whatever the unit limit on reading your speed is, and on a bike many a speedo with wheel speed sensor will read to within 0.1km/h or less than 0.5% precision, and do so with excellent consistency.
So you can say, provided you don't change tires/wheels, that making relatively comparisons with the speed measurements is still a very valid thing to do, even if the absolute accuracy sucks.
In this case, attaining accuracy is only a calibration adjustment setting away (e.g. a correct wheel circumference).
So the issue with power measurement is assessing whether you are dealing with an accuracy issue, and/or a precision issue. With high end power meters, they are generally good with precision/consistency, and also enable you to adjust the calibration so you can attain excellent accuracy as well.
But we also need to consider under what conditions of use and riding they are precise/consistent. Some struggle in some circumstances while being fine in many others.