Just in case anybody cares, here is an update on this project.  The magnet
fell off and I "designed" an improved magnet mounting "system."  Go to the
website below for details.  Other than that problem, it has worked well for
me.  I have included the original message about this in case you have no
idea what I am talking about...
Edward

Here is a project I am working on for the vanagon camper. I have adapted an
inexpensive bicycle "cyclo-computer" to become a powerful trip computer. It
senses speed from a sensor I installed fairly easily on the front left
wheel. It tells your current speed, down to .1 mph. It tells your average
speed for a trip, your maximum speed, your trip distance down to .01 miles,
the time you have been moving, whether your current speed is faster or
slower than your average. It has a time of day clock, it turns itself off
when you are not moving for a few minutes, starts automatically when you
start driving. It has a function that will scan automatically through all
of its readouts. It comes with 2 mounting kits so you can use it on 2
vehicles, or even on your van and your bike. It can switch easily from the
configuration of one vehicle to the other.  You can reset the "trip" at any
time.  It can read in 12 or 24 hour time, miles or km. You calibrate it to
the circumference of your tires, down to 1 mm, so it has the potential to
be very accurate.  It has an odometer that keeps going to measure your
cumulative mileage.

I bought the kit from Bike Nashbar. It was about $15 with shipping. The
speed sensor is basically a magnet and a reed switch.  I am currently
(4/1/02) testing it out, and so far it is working quite well. I have read
up to 75 mph on it. There is a bit of a lag in the speed display at times.
I think it only updates the speed every second or two, so sometimes it is
behind what the speedometer is reading if you are accelerating (ha!) or
decelerating significantly.

Why, you may ask, would I do this.  That is a good question.  I had a VW
GTi that had a built in trip computer, and found it quite entertaining on
trips when there was not a lot to look at.  Generally, I love information,
I'm pretty cheap, and I get a kick out of adapting a product to do
something useful other than what it was not intended for.

Here are some photos:

http://buncombe.main.nc.us/~emaglott/pp8/index.htm

Edward