Sigma Speedometer Calibration

By Lance Haysom
November 2001

Phil Billingsley is quite correct with his doubts as to the accuracy of setting up his Sigma computer. I had the same difficulty and concerns with mine.

The first thing to consider is that the circumference changes with load and speed. In fact, the greatest error will be obtained when the bike is furthest away from its real life use situation, namely, being pushed forward, at a lean, without a rider on top. These unfortunately are probably the exact same conditions used to set the computer up in the first place.

I improved on this accuracy by riding it over 10 revolutions. OK. It does need a big tape measure but it improves the accuracy no end.

To increase accuracy further I needed an accurate mile (or more). I found out that my local council has a 'measured' mile that they use for testing taxi meters. I don't know but I also assume that other uses could include calibrating police speedometers, etc.

This 'measured' mile was simply a local public road with some markings on it a mile apart. Unless you knew what they were for you'd never give them a second glance.

What you have to do is to ride over the whole mile at your typical speed and load. You should be at your typical speed when you enter and exit the mile. I'd also suggest having a half tank of fuel on board as well.

Note exactly how far your computer said this mile was. If you messed up, don't worry. Just do it again. Each time you do it just note down what the results were as all your doing is increasing the accuracy with each measurement.

The next step is just plain old maths...

We know the mile is a mile. Lets say your computer said it was 1.08. Not a big difference? Sorry, that's 8% of error!

To correct this error do the following;

The new figure = old figure divided by your computer's mileage claim


Your current figure = 1155
You did 4 runs with your computer saying
1 mile = 1.03 miles
1 mile = 1.03 miles
1 mile = 1.04 mile
1 mile = 1.03 mile
Average = (1.03 + 1.03 + 1.03 + 1.04) / 4 = 1.0325 mile

New figure = 1155 / 1.0325 = 1118

{Note; if we had not done 4 runs we'd have used 1.03 as the mileage which would have resulted in a figure of 1121}

The same principle can be used on bigger distance using GPS to make sure you get the mileage right. The only thing to remember is that it'll need to be in a straight line (or very close too). I reckon with 10 miles on the flat, the accuracy you would get by this method would be as close as the unit will allow, what with it having only whole number inputs.

Disclaimer; when riding or driving any vehicle, it's always a good idea to LOOK WHERE YOU ARE GOING! You do NOT need to stare at the computer while you are doing this procedure. A glance at the beginning and end is quite sufficient. Remember, bumping into things on a motorcycle is quite likely to hurt and make you look dumb.