Marc, lonniecdams,

The rho calculator in GC uses "station" barometric pressure. Unfortunately for us, official weather stations like those at airports standardize to sea level. Lander WY is at an altitude of 1600m, so if you're using an official barometric reading rather than "station" pressure, you need to adjust for that.

What this means is that air density in Lander was close to 1.00 rather than 1.219. Since the density was lower, it unfortunately means that lonniecdams CdA was higher. rho multiplies the CdA, so if rho is lower by 1%, then CdA is higher by 1%. In this case, rho appears to be about 17% lower, so CdA will be 17% higher.

(As an aside, a quick rule of thumb is that air density decreases by about 1% for every 100m in elevation gain, so since Lander is at 1600m ASL, the rule of thumb is that rho would be about 16% lower. The rule of thumb isn't bad).

In any event, the difference in the CdA between the two tests is about .008 m^2. That much difference in drag area is roughly half way between the drag of a standard business card and a 3x5 index card; or, using Coggan's rule of thumb, at racing speed that's close to 30 seconds in a 40km TT.