i just got a cateye astrale 8 cyclecomputer for my fuji team sl road bike. i put the computer on and while going through the functions it says to set the wheel size in mm, thesize on the tire gives two sets of numbers. 23-622 and 700 x 23c. i set it in the cyclecomputer as 2300. the stock setting was 2096. was i right?
No. The default setting (2096mm) is for a typical 700X23c tire size and a typical rider's weight.
However, one company's 700X23c isn't necessarily exactly the same circumference as another company's 700X23c. It depends on tread height, etc.
The circumference will also depend on the tire pressure and the rider's weight.
For the best accuracy, you should do a "rollout test". Here's how...
1. Pump up your tires to your usual riding pressure.
2. Put a mark on the sidewall of the rear tire and a corresponding mark on the ground where the mark on the tire is at it's lowest point.
3. With help, sit on the bike like you're riding and roll out a number of revolutions (the more the better, but don't really need to do more than 5) on the rear tire. Make a mark on the ground where the mark on the sidewall is at it's lowest point. Make sure you start with the two marks (sidewall and ground) lined up and that you ride in a straight line.
4. Measure from your initial mark to the new mark in inches.
5. Multiply this measurement by 25.4 to convert to mm, then divide by the number of revolutions the rear tire made. The result is the actual tire circumference at your riding pressure with you riding.
Here's an example...
I weight 205lb and use 700X23c Panaracer Stadius Pro's at 125psi. With no rider, the circumference comes out to 2106mm. With me riding the bike, it comes out to 2086mm.
As you'll notice, the default (2096mm) falls right in the middle of these two measurements.
Wether it's worth your time to go through this procedure is up to you. For me, the difference only ends up being 10mm (1cm) per revolution. Probably negligible. But, hey, I'm an engineer, so...
If you're not religious about your tire pressure, I'd just stick with the default of 2096mm.