Speedometers are allowed to be +/- 5%. Most manufacturers err on the +5%. This may sound like an advantage since your real speed is lower than you think. But the odometer also will be +5%. this is great for manufacturers and shops, because your calculated mileage (if you manually track) will look 5% better, your lease miles will run out 5% faster, you do maintenance 5% more often, and you are more likely to replace your car because your car seems to be 5% more worn.
I have one of the very few cars with kind of accurate speedometer (within 1 mph of GPS, and those radar devices that tell you your speed). but most cars will show about 5% higher speed than actual (same GPS used)
I agree, at relatively high and constant speeds, like a car, and relatively long distances with the same speed, the 15ft inaccuracy will most likely cancel out. But a human-powered boat will change speed constantly and at low speeds the % error will be greater. Unlike a car, a kayak also zig-zags and accelerates and decelerates with each stroke.
I think the whole idea isn’t feasible otuside a lab. But it gave me the idea to use my cheap bicycle GPS computer to monitor my approximate speed over ground. With wind, waves, and currents it is hard to tell if I’m actually fast over ground, or just fast compared to waves.
