calibrating a speedometer

[throbenotoid]

The speedometer on my Super III seems to be running about 10 mph too fast while I'm doiing 60-80 mph. The front wheel is original, so that's not the issue. I've read that automobile speedometers can be re-calibrated by switching out the gear in the back of the speedometer, which can be obtained at an auto parts store. Anyone know if this is the case with a motorcycle? If so, do I need to source the gear through Triumph, or can I bring the gear to an auto parts store and attempt to match it?

thanks

[JohnD]

Never heard of such a gear, but I'm all for more accuracy.

Under U.S. law, manufacturers are prohibited from speedometers UNDERestimating speed. So to be sure they comply, manufacturers routinely set speedmeters to show slightly higher than actual speed.

I have a Pontiac that is 3-4 m.p.h. optomistic. My Audi is actually spot on. Motorcycles seem to be more inaccurate. I have always suspected that is because the motorcycle manufacturers want owners to think that their model is faster than it is - plays to the "boy racer" attitude of many riders.

[KitNYC]

I've never heard of swapping out a gear either, but your speedo sounds a lot more optimistic than most. My RS reads about 7% fast, and most bikes are right around 8-10% optimistic from what I've read. What are you checking your speed against?

Cheers,
-Kit

[sailfish]

Mine '98 Sprint speedo is optimistic by about 10%. Put a bicycle computer on your handle bars and you'll never use the speedometer to regulate your speed again (except at night). Get this one and get it here. You might find a cheaper one but this reads to 186 mph and has a wire that is long enough.

http://www.aerostich.com/sigma-speedometer.html

It's highly accurate and can be calibrated to your tire. It keeps track of a lot of travel data too. It's the best thing that I've bought for my Triumph.

[throbenotoid]

Yeah, well from what I've read, speedometers on non-American bikes typically are 10% in error on the high side, because the manufacturers would be exposed to hefty government fines if the speedometers erred on the low side. Obviously, there's no easy fix on electronic/digital speedometers, but I gather that low-tech automobile speedometers that run off a cable have a hairspring on the back of the speedometers that holds a gear in place that can be changed out specifically for this reason. I'm going to remove the speedometer on my Super III and see if such a gear exists. If it does, and if I can remove it, I'll take it to an auto parts store and see if I can remedy this problem. It's pretty annoying really....I think the error is a bit more than 10mph off, actually. The way I noticed was that I was driving next to some new Japanese car on the highway whose speedometer had a big digital display that was easy to read. Just to be sure, though, I'm going to keep my eye out for one of those police displays on the side of the road that measure the speed of an approaching vehicle. I'll keep you guys posted, if you're interested.

[JohnD]

Throbenotoid,

Nice find on that Sigma speedometer. Great for Florida where at 10 miles over the speeding ticket leaps to big dollars, and more points on your license.

A GPS unit will also give accurate speed if you need directions too etc. Of course is much more money.