Originally Posted by meowy84
As to the car speedometer, I hope you weren't being entirely serious.
I know I may be splitting hairs here but I don't think that that particular method of measuring speed is entirely precise either. Sure, it can give a rough hobby level idea of the speed but it is not at all uncommon for car speedos to have an error margin of +/- 5mph or more. Then there's other factors too on top of that...
Ofc you're right - but the filmed chase car's speedometer is a compromise for those that don't have a GPS or access to a track and it helps to reduce the potential for "clever" bicycle speedometer shenanigans. While a car's speedometer is most definitely NOT a precision instrument, it's not as prone to such easy abuse as switching the MPH to KPH and then sayin' "ZOMG I did 47" or whatever lol
Here's a previous lil rant on the subject of car speedometers
Originally Posted by BarelyAWake
It's commonly accepted that even the factory speedometer in a car has a comparatively large margin of error: Speedometer error, Wikipedia
whereas even the basic modern GPS has only about a 0.1 knot (about 0.1 mph) variance if tracking four satellites: Global Positioning System, Wikipedia
Throw in the problem that it's simply impossible to have a "correctly calibrated" bicycle speedometer with the fact that when discussing motorized bicycles we're often bickering over even 5mph increments - such speedometers are really only good for "ballparking" speed estimates
at best. Even the "radar trap" signs aren't nearly as accurate as we've been lead to believe (target size and vibration have huge effects): Radar gun, Wikipedia
, to the point where speeding tickets have been contested successfully with GPS data: GPS Data Used To Disprove Radar Gun In Speeding Trial
For just $10-20, sure a digital bicycle speedometer is a fun toy to give ya an idea of your speed - IF you can manage to get it set even vaguely correctly, but remember the faster you go - the larger the error, whereas the opposite is true with a GPS as it'll get more and more distinct tracking information beyond say a slow walking speed (about 3mph).