The fault lies in the failure to calibrate the device.

First, let me dispel any possible confusion. To calibrate something is not the same thing as setting it. When you set your computer, you're simply entering a number which you think is accurate. Since the device uses that number to make all it's calculations from, it's overall accuracy obviously relies on the accuracy of that number. But, since you haven't tested the accuracy of that number, who knows?

When you calibrate your computer, you are testing it by comparing it's readings against measurements you know to be accurate. Then by making the appropriate changes in the setting you make sure those readings are accurate. As much as possible, at least. In the end, you'll not only be sure your bike's computer is reading as accurately as it is capable, but you also know exactly how far off those readings are!

So how do you calibrate it? to what do you compare it with? Well, you could get a tape measure and wrap it around your bicycle's wheel. But at best, I've only gotten an accuracy of 95% with that method.

Why? Well, first, you're only measuring one wheel turn. And your wheel makes almost 800 turns per mile. And with every turn, any error, from any of a number of factors, will be added and multiplied over each and every turn.

Obviously then, the longer your measuring stick is, the better your results. Because there are a number of factors that affect you wheel's "size"

• Tires
• Make and model of tire (even if the size printed on the side is the same, the tires aren't)
• Inflation pressure
• Tread wear
• Weight
• You
• The bike
• Other cargo (like panniers)
• Riding style
• How straight a line do you ride?
• Your position on the bars
• How often you change that position?

So how do you take all these factors into account? Easy, ride the bike over a known distance that you know to be accurate. Then you can compare your comp's measurement with the actual distance ridden, and make the necessary corrections.

So where do you find a distance of known accuracy? Well, is there a track in your neighborhood? Either a velodrome or even a running track will do These are usually a measured distance. Those that are used in official events, like the Olympics or even collegiate events, for example, are best, because they have to be precise, official records are made and broke there.

Simply divide the distance you want to test against, say ten miles (or kilometers), by the distance of the track. That will give you the number of laps you need to ride. But what if there isn't such a facility in you area? Well then you'll just have to make one.

At this point I should interject, do not use the mile markers on the side of the highway. Thay don't even agree with each other, let alone the actual distance they are supposed to represent. At best, they are reference points for the road workers, nothing more.

To make your own measured course, go here.

If you already have one, read on.

First, set up your bike the way you normally do when you go on a ride. Use your regular tires, inflated to their usual pressure. If it isn't already, set your computer with what you believe to be the correct wheel size. Select the "trip distance" mode and reset the mileage to 0.00. You may also wish to take a calculator with you, as you will be doing some math.

Place your front wheel on the starting mark, and start riding, Ride like you normally do, changing hand positions as usual. And don't try to ride "as absolutely straight as you can". Remember those factors? One of them was riding style, so you want to take that into account. Just ride like you were on a regular bike ride.

When you approach the end of the distance (let's say it's 10 miles), slow down and stop with your front wheel on the finish line and check your computer. Don't be dissapointed if it doesn't say exactly 10 miles, it probably wont. Now get out your calculator...

Ready for the math? Divide the actual mileage traveled (in this example 10.00) by the distance your computer claims you traveled. For this demonstration, let's say it reads 10.06. That means your computer's set to high. So to find our "correction factor";

10.00/10.06 = 0.994035785

...or thereabouts, depending on how far you calculator displays. This is the number you use to correct your bike computer's setting, so write it down or put it into your calculator's memory.

Now, to make the correction. Go to your bike computer's set up mode and find the wheel size that it's currently set at. take that number and multiply it by the correction factor you just calculated. let's say, again for example, that it's set to a wheel circumference of 82.78". So;

82.78 X 0.994035785 = 82.28628231

that is the correct wheel circumference that should be set into your computer. Obviously, you cannot force all those decimal places into it, so you must round it out to whatever's nearest to a number you can set in. let's say that 82.28 is the closest setting number for your computer. Enter it in.

You can always check this, of course, simply by re-riding the course and checking the new readings. Who knows? You may even be able to get it closer.

Here is the complete formula, so you can take it with you;

W = w(D/d)

OR: Correct wheel size (W) equals the set wheelsize (w) times the result of the actual distance traveled divided by the computer's distance reading (D/d)

With this information, you can also calculate the final accuracy of your bike computer's readings. To do that, take the number your calculator said you should use and the number you actually entered into your bike's computer, in this case, 82.28628231 and 82.28, and divide the smaller number by the larger. Then multiply the result by 100;

82.28/82.28628228 X 100 = 99.9923653

...Your computer is now at least 99.992% accurate. At least in this example, it is. That's a loss of only 0.08 miles for every 1,000 traveled!