IBike and IBike software give different wattages

[GWPOS]

I just did another 20 min power test...ok 19:30, ran out of road. My IBike unit said I averaged 296 watts, but when I downloaded it to the IBike software it changed it to 305 watts. Normally I would not care so much but since I am using it for my FTP test and future workouts I do I would like an accurate figure. So which one do I use? I attached my test file also.

iBike_12_24_2010_1427_24_Miles_OakGlen.csv

(1.35 MiB) Downloaded 361 times

My last test was 290 watts which came out to an FTP of 275 which I did in the beggining of October. Since then I broke my wrist and was stuck on a trainer for 4-5 weeks doing 3 hard workouts a week mainly 2x20's, but I still find it hard to beleive I have a 15 watt increase according to the software.

[Velocomp]

The ride file posted was analyzed by the iBike software. This happens one of two ways: 1) you use the command Tools/Analyze Route... or 2) the box is checked in Edit/Preferences/Analysis/Automatically Analyze Route.

I used the command Tools/Return to Original Download... to return your file to its as-downloaded condition.

I returned your ride file to its original state, then applied the Analyze Route function, the watts changed from 296W to 302W. This is a 2% difference, which is within the measurement error bounds of the iBike. I don't know which one is more "accurate".

The most important thing when comparing test results is this: consistency of operation. If you "analyzed" your previous ride file to get your fitness wattage, then do the same thing here. If you did not use the Analyze function, don't do it here.