Fofa, Rich is right. Think about it. If you take two mountain bikes with 26" wheels and put a 1" or 1.5" slick on one and a 2.4" downhill tyre on the other. You are obviously going to get wildly differing circumferences.

Another way to measure is to unroll a roll of wallpaper on the floor, blank side up. Draw a narrow line across it at one end with kids poster paint. Now get on the bike and ride along the wallpaper. Your tyre will pick up paint and draw a second line about seven feet from the original line. Measure the distance between the two lines. Do it with both bikes and I'll bet there's a difference of a few inches between the two .