Bike Computer Wheel Circumference Chart(s)
 

Hmm ... scratching my head cuz the chart that came with my computer doesn't jive with Sheldon Brown's and also I don't understand why a 27 x 1" wheel tire combo would be different than a 27 1/4" wheel tire combo.

I tried measuring my wheels' circumferences but it is too awkward with a metal tape measure, so i'd like to use the chart. I've been riding different bikes with different wheel/tire combos and I'll be using yet another tomorrow, as well as changing a tire size on a current bike. I want to get this right and also want my personal performance comparisons to be accurate. Hope that makes sense.

OK, 1st issue chart differences:

My computer's chart:
27 x 1 = 2136
27 x 1 1/4 = 2155
700 x 32c = 2155
700 x 28c = 2136

Sheldon Brown's chart:
http://www.sheldonbrown.com/cyclecomp_f.html

27 x 1 = 2145
27 x 1 1/4 = 2161
700 x 32c = 2155
700 x 28c = 2136

So it looks like my settings for my 700 x 28 tires will be fine cuz both charts match. But I'm concerned about my 27 x 1" setting. Should I set 2145 or 2136?

I'm reluctant to use the rollout method cuz it's dark and rainy out and I don't have any chalk.

2nd issue - why would the circumference measurement be different between the 27 x 1 and the
27 x 1 1/4???

Its a whopping 0.4% difference between the two. That means if you're rolling at 20 mph it will say you're doing 20.08. IT DOESN'T MATTER.

LOL! seriously? that is too funny

Yes, and the reason the circumference is larger for a 27"x1 1/4" is because its thicker on all sides, not just parallel to the axis of rotation, so the radius is larger.

Also, you're not considering that the tire deforms under load, etc.

How exactly is this awkward? Your not trying to bend the scale around the wheel are you?

Take bike out of garage. Find nice flat surface. Set wheel on surface with valve stem at 6 o'clock. Mark surface beside valve stem. Roll wheel until valve stem is at 6 o'clock again. Make second mark on surface. Put bike back in garage. Pick up awkward tape measure and go back outside. Lay tape measure nice and flat between the two marks on the surface (about 7 feet apart). Take reading and convert to millimeters. Take millimeter number and plug into cycle computer. Feel proud when telling everyone you accomplished a very awkward task !!!!

+1
Triple roll-out and average the measurement. I actually use a line of stamp-pad ink on my tire, and ride it for a couple revs of the tire, that way I get the "true" measure of the flattened out tire under my weight.

And on my 20" x 1.75" Sheldon's page says 1491, the Astrale 8 manual says 1515, and I have wound up at 1545. What I have done to arrow down my actual number is ride sections of the bike path that have mile markers posted. Then I just modified the entry in my computer until it came up correctly riding that path.

I have since double checked my numbers by mapping my normal commute using http://www.mapmyride.com/ and zooming in to make sure my map route really follows my actual road route. Over the 6.5 miles, the difference between the two modes (map and actual) is only ~100 feet. Definitely within my personal level of error.