Because Odette and I were getting different distances on the same ride (like a 0.25 difference on a 30 mile ride) I finally rolled out several of the bike computers and compared them to the tables published by their manufacturers.
| Bike | Rim | Tire | Inches | Millimeter | Specialized | Veta | Filzer | Cateye | |
| Jerry - touring | Continental ultra 2000 | 700 c | 28 | 83.5 | 2120 | 2134 | 2136 | 2136 | 2136 |
| Jerry - mountain | Serfas drifter | 26 in | 1.95 | 79.5 | 2019 | 2074 | 2055 | 2099 | 2050 |
| Odette - touring | Serfas barista | 26 in | 1.25 | 77.0 | 1956 | 1962 | 1953 | 1953 | |
| Odette - mountain | Serfas drifter | 26 in | 1.5 | 78.5 | 1994 | 2010 | 1985 | 2030 | 2010 |
| Will - touring | Avocet cross | 700 c | 32 | 2249 | 2155 | 2155 | 2155 | ||
| Will - mountain | Specialized fat boy | 26 in | 1.25 | 1962 | 1953 | 1953 | |||
| Tandem | Avocet cross | 700 c | 32 | 83.5 | 2133 | 2249 | 2155 | 2155 | 2155 |
Most of the Specialized values are ones that were pre-programed into the computers and are described as reflecting the sizees of Specialized brand tires. The Sefas tires aren't knobby - they have lightly indented tread - so it makes sense that they would be smaller than an ATB tire of the same size.
Here's a link to Sheldon Brown's cyclometer calibration page.