I actually have fully working original odometer & speedometer on my '87 Westy and had the same on my '85 (reading the list this may seem a little unusual, but then the listos to discuss problems...). 

This has allowed me to check the calibration of my Garmin GPS unit (model C330).
- this model has both a speedometer & trip odometer (hidden in a back menu).

I find that it reads very close to the odometer reading (and to Yahoo or Google maps), and that the speedometer feature is also accurate (so long as you are not accelerating or braking hard - not a common worry for a 2.1l powered Westy) to the point of confirming that the vanagon speedometer reads high by
~3mph at 70mph (I have 15" rims with Michelin Agellis 205/65R15 tires).
I have checked this over several long trips to the sierras and up to Oregon on I-5 (from here in the SF Bay Area). The mileage remains accurate even for windy mountain roads (perhaps a small error when you loose
satellite contact in tunnels or trees, but even then the GPS keeps trying to calculate distance based on the
last measured speed).
Since GPS systems can be purchased for as low as $130 now (try www.tigerdirect.com), that may well be a less expensive (& certainly less work) solution to fixing/replacing your current broken odometer/speedometer
and to running out of gas
BTW, you can keep the GPS trip odometer running over multiple trips just by leaving the power on.

Mike
'87 Westy

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