I'm not sure what 'L' is, but as mentioned you can use the d or r in case you need to calculate the circumference which is calculated as C = 2 x pi x r or C = pi x d, where pi is a constant number of 3.14159...
Or, if you need to measure out the circumference manually, put a chalk mark on your wheel, get on the bike and align the chalk mark (at the bottom of the rotation) with a line on the ground then roll your bike as you are on it to a point where the chalk mark is again at the bottom of the rotation. Measure the distance between the two marks on the road and that will give you the ground distance for a full wheel's rotation. Really, one way or another that is what your computer needs, so you can calculate the r or d from that manual measurement.
Specialized - Allez and TriCross Expert. Cannondale F900SX MTB. Kona Unit 2-9 Single Speed MTB. Tern Verge X20
Thanks B, yes I need the circumference. I won't have the bike for a few days. I just wanted to set up the computer now and thought I'd check if anyone had the number instead of waiting to measure it myself.
You will get the most accurate performance from your computer if you use the actual circumference of the tire at the inflation pressure you will use with your weight on the bike. But even unladen, the measurement is still better than a number of a chart. There are differences in tire circumferences for nominally equal tires.
I use the method that brentirvine suggests with the difference that I use three rotations and then divide the number by three, this presumably compensates for measurement errors as it gives you the average of three rotations.
For most computers the input number is the circumference in millimeters. Hope this helps.