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That's a creative idea. My phone actually has a stopwatch function that measures hundredths of a second, and the screen is a lot brighter than a stopwatch LCD. I will certainly try it next time.
Your caveat, KamperBob, is disconcerting, though picking the specific frame where the needle is exactly over a given speed mark is probably a far greater source of inaccuracy. Like I said, a given measurement has up to 4 or even 5 frames that look about right, which means each measured interval between 2 speeds can be up to .17 seconds off, or about 2.75% over 6 seconds. That's significant, but probably not more so than variables like wind gusts and pavement friction at different widths of the road (the latter is something I'm going to be more careful about next time). However, I don't know how that range of error would translate into Cd and mpg figures.
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