> although my GPS seems to provide accurate speed, it ends up way off on
> distance, usually many miles low over a couple of hundred miles.
>

Are you sure it's the GPS reading low? Unless you're measuring against
a known surveyed distance, it's hard to get more accurate than GPS. If
the GPS is getting the speed right, then the distance naturally
follows, given that it calculates the former by knowing the time it
took to travel the calculated distance between the previous "fix" and
the current one. Unless you're dealing with signal dropouts from
tunnels or trees causing it to "cut corners", I'd wager the GPS
reported distance is accurate to within a mile.

--
John Bange
'90 Vanagon - "Geldsauger"