An alternative procedure (also useful if you ever make cue sheets for a ride) is to first accurately measure the distance on a map. On Google, go to Google Map
's My Maps by clicking on "My Maps" in the upper left area. When this comes up, there is a list on the left titled "Featured Content". Go down this list and click on "Distance Measurement Tool". On the following menu, select miles or kilometers (kilometers has some advantages, see below).
Pick out some route that is relatively level, nice to bike and at least 2 or 3 miles long. The straighter the route the easier it is to use the Distance Measurement Tool but it can be used even on winding bike paths if you can see the path using the Satellite view of the map. Working at the highest or next to highest detail setting, click away on your route, using lots of clicks to follow any curves. Pay attention to exactly where you click on the road because your measured distance will change if you start on one side of the intersection or the other. If visible on the Satellite view, I used sidewalks as precise beginning and ending points. There is some advantage to using kilometers because the two decimal places you get is a more precise measurement since 0.01 Km is a smaller distance than 0.01 miles.
I do not know if Bing Maps or Mapquest etc. have a similar measurement tools.
Now pump up your tires to whatever you typically use and load up your bike if you usually travel with a significant load. For the best precision, set the cyclocomputer to measure in kilometers. Go to one end of your measured route, reset your trip meter and bike the route. Stop the trip meter when you exactly get to the other end or write down the exact distance your cyclocomputer records.
Get a calculator (your cell phone's or whatever). Get the value for wheel circumference the cylocomputer used (often the cyclocomputer instructions give a button combination to display it). Calculate the following:
(circumference in the cyclocomputer) * (true map distance) / (cyclocomputer recorded distance)
The result is the true, effective wheel circumference at that tire pressure with you + any load on the bike. Enter this new, calculated, effective wheel circumference into the cyclocomputer. If you switched the cyclocomputer to kilometers, put it back to miles (at least my Cateye and Planet Bike cyclocomputers will convert your odometer reading etc. from miles to kilometers and back to miles without a problem).
The advantage of this method is it gets the effective wheel diameter under load and as you bike in however a straight line you bike at your typical biking speed. It has worked better for me than slowly biking ( and thus some wobble), counting wheel revolutions (typically 10 or so) and getting out the long measuring tape. It will also now match any cue sheets you make off a map.
I've checked the Google Map distance measurement versus software that uses US Geological Service topographic maps to measure distances including any up and down the earth's contours (which Google Map's measurement does not account for). As long as your route is fairly flat, the Google measurement agrees almost exactly (+/- 0.01 miles at almost 3 miles).
p.s. I originally used a slightly different Google Map tool, the Distance and Area tool. The Distance tool described above gives distances to five decimal places, so no need to change units to kilometers.