1) Ride off a really tall cliff. Of course, then yo need something to determine air speed, since your wheels are no longer touching the ground.

2) Report you speed in kilometers. Then you can report things like "I averaged 30+ for a two hour ride" (about 18 mph).

3) Ride straight down a double black diamond (ski) run at Vail. Similar to problem in number one. You'll build up great initial speed, but once you start bouncing down the mountain end over end for a couple thousand yards, your average speed as recorded on the computer will drop. Just remember to stop the computer just before you totally lose control. That way, if both you and your bike somehow survive, you'll have that high mph data from the beginning of your run. Oh, yeah, get a helmet-cam, and put a note on it..."Please post this film on BF"