Consider a handheld GPS unit. Yeah they're more expensive but they do so much more. I have a mount on my handlebar that the unit snaps in and out of. I set-up the display to show: speed, max-speed, odometer and trip odometer all at once. I also have it set up to record my track as I'm riding. Every 30-50 ft. it records a point that includes position, speed, elevation etc. Later, you can upload the track to a PC and see where you were, what the trail looks like and how fast you went. I know this seems like overkill, but if you have other uses for a GPS then I think it's a great way to go.
I bought a Garmin e-trex Legend after reading a review in REV magazine. I got it at Sports Authority on sale for $209.00 with a $50 rebate from Garmin.
www.garmin.com
Matt