For those of you who ride 20 x 1 1/8" (451) tires and use the MC100, what setting do you use for the computer in order to correctly calibrate distances, mileage, speed, etc? I noticed on the instruction sheet a setting of 1515 for 20 x 1.75 tires and 1615 for 20 x 1 3/8 tires, but nothing for 20 x 1 1/8 tires. Is it a linear extrapolation or is there a simple formula to use for deciding on the setting?

I had the same problem with my folding bike.

I used linear extrapolation and it seems to be pretty ok.

I got like 42.68 miles for the 5-boro bike ride.

I use 1587 for Schwalbe Stelvios in 451 size. I got this figure with the rollout method - sitting on my bike and rolling across my dining room floor! It should be pretty close - within 5 mm - of the actual circumference.