I just received numbers from a new measurer this morning. His working constant was 9767 cts/km based on his pre cal rides,done at 45F. His finish constant based on his post cal rides was 9774 cts/km. The temp was 67F that afternoon. Other than a slow leak in the front tire can anyone come up with an explanation for this?
Original Post

Maybe the post cal ride wasn't as "straight" as the pre cal ride? Be interesting to see the data from all eight cal rides to get a feel for the data spread. Your hypothesis of a leaking front tire is probably the best however.
An unlikely, but technically possible explanation, would be if the front wheel was rubbing the brake during the pre cal, but was running completely free during the post cal. Increased resistance to the rolling of the front wheel has the effect of INCREASING the effective rolling radius of a pneumatic tyred wheel.

It is unlikely because quite a bit of friction would be required for an increase in calibration constant of 0.1%, and it would be noticeably hard work to ride the bike. The rider would have surely got off and adjusted his brakes.

A leak is most likely. Ask the rider to go out and calibrate his bike again today, and by the way report back if the tyre is flat.
If it is not due to a very slow leak and the best experts cannot come up with another explanation, then the data may be wrong. Perhaps the readings were incorrectly recorded or the post-cal data was really the pre-cal data.

I hate to suggest this, but perhaps the data was changed in some way for some unknown reason.
What about the course measurements Tom? Was the second one shorter than the first?
Perhaps the bicycle was kept inside a warm home and calibrated before the tire had a chance to acclimate to the cool morning air. Or, maybe the afternoon temperature was cooler than recorded. It may be his thermometer was off because it was sitting in the sun in the afternoon. That might happen if he used a bicycle odometer (like the Protege) with a built in thermometer.
Thanks for all the input. Unfortunately I don't have the work sheets but I do intend to talk to the measurer soon.