And some new ones too!
To tell the rider what speed the bike is doing involves a speedometer being driven off something that is directly connected to the road. The only things that touch the road while you are riding is the tyres and that is the problem. New tyres are a bigger circumference than old tyres and different makes can also be different. Then there are sprockets that drive the wheels, they may have been changed to give different gearing, this would not matter if the speedo drive was off the front wheel and that wheel still had a standard sized tyre fitted. For all of these reasons all bike speedos are designed to over read by up to 10%. If the speedo had been accurate with worn tyres at 100 kmh and then new tyres were fitted, the speedo would be showing 100 kmh while the bike was actually travelling at 110 kmh, speeding fines here we come.
The problem with the small speedo fitted to a 50 year old Ducati 250 that i happen to own, is that the vibration will swing the needle back and forth so that when travelling at 60 Mph (yes it is in mph), it indicates somewhere between 35 and 65, and I would need to ride with reading glasses on to see that. The other problem is that all of the cables from the handle bar controls want to pass the speedo face, and you can bet that with a bit of vibration they will find their natural place – across the middle of the dial!
The most accurate speedo that you could use is a GPS driven one, which will show you a very accurate speed at all times, whatever tyres or sprockets were fitted to the bike. Many of us have a GPS speedo in the form of a smart phone or tablet. I have a small Android phone that I use with an app called “GPS Speedo”. The app is free to download and looks like this:
Okay this picture is taken while doing 63 kmh in the car, but a strip of stick on Velcro on the back of the phone will let me attach it to a bike too. It is far more visible than the bike speedo, and even tells me an average speed in the bottom left corner of the screen.
It would be great is someone in China would make a small screen like this with a GPS receiver and an Android operating system, but no phone, SIM card, memory card or camera bits inside, to use as a GPS speedometer. It would not need to be expensive, after all the phone in the picture above was only a €59 item. Perhaps someone does.