Originally Posted by Cyclaholic
you could use a data logging wind speed gauge, ofcourse you also need to correct for variations in air density caused by fluctuations in barometric pressure, temperature, altitude, and humidity so you need to datalog those variables. As well as speed you also need relative wind direction because the drag factor varies with a crosswind, most of these variables can be datalogged with instrumentation used in yacht racing, any aerospace contractor would also be able to help you.
Ofcourse, carrying these instruments and their power source will add weight and aerodynamic drag so you'll have to develop a computer model that will correct for that before you can get your true performance. You'll need to establish some baseline performance measurements under controlled conditions to develop an accurate model, see if NASA can help, they have the wind tunnel and necessary instrumentation.
Let us know how you go.
Cyclaholic is right. You also might want to put the measuring instruments somewhere in front of or above the bike (on a rod), since your body and bike will be pushing air and thus affecting the measurements. Getting useful information from the measurements will require some deal of (interesting?) computing, because of various nonlinearities in the effect of windvelocity, apparent windangle etc. on drag.
Regarding bikecomputer v.s. GPS units, I think that the major difference in accuracy of speed measurements are:
1) sampling rate. Bike computer samples once per rotation of wheel, but this is adjustable for GPS units (but I don't think that a normal GPS devices offer high enough sampling rate to outperfom the bike computer).
2) bike computer has to be calibrated to tiresize and pressure (no calibration needed for GPS) and quite many bike computers out there are not calibrated correctly (but those who wan't data with high accuracy probably do calibrate there bike computers carefully)