But you knew that already, right?

We’ve all been there. You roll past one of those flashy “YOUR SPEED” signs on the side of the road, and the number confuses you. You hook your GPS up to your bike, and you notice a significant discrepancy. Are you crazy?

Well, I can’t tell you that you’re definitely not crazy, but yes, we’ve all noticed that our motorcycle speedometers can be anywhere from slightly to extremely unreliable. This question comes up in bike groups all the time, but never seems to get resolved. Is it because there are always new people to the sport? Maybe most riders don’t pay any attention to this and just assume their speedometer is correct?

I’ve ridden a lot of motorcycles in my life. Probably forty or fifty, and only a couple of them had accurate or mostly-accurate speedometers. You might know I’m all about metric bikes, and that might be why my experience is thus. The bikes I’ve ridden that had reliable speedometers were all American bikes.

Mostly, I’ve found the speedometers on the bikes I’ve ridden and owned to be around ten percent optimistic. I got a great lesson in this particular subject when I put new tires on my Kawasaki KLR. The  worn-thin Avon Gripsters I was running didn’t help the speed discrepancy on the bike, but when I mounted new, much knobbier tires, that speedometer discrepancy went away, and the bike then had an accurate speedometer! Aha!

This discrepancy, according to Cycle World, “can most likely be traced to ECE Regulation No. 39, which is a 14-page document detailing speedometer accuracy for vehicles sold in EU countries. According to this law, a speedometer can read high by as much as ten percent plus 4 km/h at a specified test speed, but under no circumstances can the speedometer read low.” It’s a classic case of CYA for manufacturers who sell motorcycles mostly in the EU, which is most of them.

There are products on the market like the Speedo Healer which purport to, well, do what it says on the tin: heal your speedo, and make it more accurate. The problem there is, it will read differently depending on how worn your tires are, how much air they have in them, what size wheels you’ve put on your bike, etc. It’s harder to get a motorcycle speedometer to read correctly than a car’s, and the motorcycle manufacturers are hedging their bets to stay on the side of legality when it comes to regulations where they sell so many bikes.

So what is the best solution? The ton of products on the market aimed at curing this discrepancy are as subject to change with tire condition and wheel size as the stock speedometer. You can mount a GPS on your handlebars, and with its satellite positioning it will give you an extremely accurate reading. Your phone, too, can tell you your exact speed.

Me? I just note the discrepancy, do some math, and ride my own ride. There aren’t enough situations in my riding habits to worry too much about it. Forewarned is forearmed, though, so you now can have it out on motorcycle forums and chat rooms with the best of them.

Source: Cycle World, Super Street Bike