Jack,

Great explanation. I knew the octane/elevation relationship, but
your short description is better than one I could've made.

I figure that I can go at least to Lincoln, Nebraska, on a tank
of gas (about 600 miles). I think the elevation there is about
1100 ft, so that would be a good time to get the "regular" 87
octane. I'll see if I can monitor any improvement/degradation
in MPG and performance as I go down in elevation with the
85 octane.

There'll be other factors besides the octane, of course. The
tire pressure should change. The air becomes less thin, so
I should experience more drag. Plus, the speed limits should
be really high on the interstates, compared to the 60mph I
drive on my commute.

Thanks,
jinno