Theoretically, shouldn't your odometer and speedometer be off by the same factor? Why not just drive a fixed distance regardless of the speed, monitor your odometer, and do the math from there? Which would you prefer to be more accurate, the speedometer or the odometer? Do you have reason to believe that they're out of whack with one another?
Even so, I trust my Garmin(a modern one) enough to give me an accurate instantaneous speed measurement. There is about a second of hysteresis on my GPS, but if you're maintaining remotely constant speed, I think the lag would be negligible. I don't think a stopwatch would be the way to go.
On second thought, I do have a watch I would use. It's also a Garmin GPS, however.
You are on the right track with trying to hit "75" on the speedo to get a reference for calibration. Getting the full-scale reading always reduces error, assuming the linearity of the transducer is reasonable. Otherwise, you'd want to get a reference where the speed would matter most to you, like "55" on the speedo.
