mattsz wrote - Someday I'd love it if you'll tell me about why it is that sea-level mileage should be worse than high-altitude mileage. Another time.
It's pretty simple really. The sea level barometric pressure averages 29.92 in/hg. The optimum fuel mixture 14:1 air to gas.
The barometic pressure decreases at a rate of about 1 in/hg per 1000' elevation increase. At 9000' the barometric pressure would be down to about 21.4".
In air pressure terms, the pressure at sea level is about 14.5 psi. At 9000' it's about 10.5 psi.
As you gain altitude and the pressure decreases, the fuel flow needs to reduce to retain the 14:1 ratio. With a good fuel injection system that is taken care of automatically by the O
2 sensor. With a carb, as you gain altitude, the bike just runs richer until you need to change jets.
However,as you gain altitude, the lack of air starts taking a toll on your horsepower, so at 9000' you can only get about 75% of your sea level power. With the fuel injection, this will cut your gas flow considerably as the air pressure reduces. Even with a carb, since there's less air, it won't pull as much fuel though the jets as it does at lower elevations. Hence better fuel mileage at altitude.
Airplanes thrive on this as the higher they go even with the big power drop, the drag drops even more, and the planes go faster than they will at lower altitudes while burning less fuel.
Jets at 35,000 feet burn about 20% of the fuel than they do down low and can go 200 mph faster. That's why turboprops usually fly the short runs, because jets are very ineffecient at low altitudes. The propellor engines work better below 15,000 feet.
Hope this helps.
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