Cavy, your numbers made me think that I ought to do the old E6B thing. My home base is 4700'. On a typical trip north through WY I'll cruise between 6500' and 7500'. ASI usually indicates 85mph, on a calm day (rare) the GPS will usually indicate 83 knots. I've always kinda ignored the mph/knots difference but it actually works out. The handy round calculator says that 85mph indicated at 6500MSL is 95mph. It also says that 83 knots is 95mph so the guage must be working. At 9500, 85 indicated would be 100 true, at 10500 it would be 102 true. That's with a Sensenich 7456 also, turning 2475-2500rpm, burning around 7.8gph, leaned a little less that a full fingernail. Haven't noticed much difference between 8.5x6s and 26" blimp tires -- maybe 1 knot. My bird is basically a stock 150, no VGs, the gear legs are standard length and not covered (this might account for some of the speed difference if your legs are covered), just safety cables hanging in the breeze. 95mph, whoo-ee, I never calculated I was going so fast!
