Mark,
70 is done, but i don't try to sustain that speed for long.  i generally
cruise in the 60-66 range.  i wish i had a tach to know more, b/c the
tire change also changes speedo. i am assuming a +3-4 mph change vs.
indicated so i try to keep it under 65 on speedo.
I know this is not the best or quality way to changing the gearing
problem, but damn good for a less than $200 and maybe 20 min labor, if
that.
when i first thought of this I played with just the back wheels. kind of
cool dragster looking with tall tires in back and small up front. kind
of "mystery machine" looking.
jeff r