Fuel mileage at higher altitudes

[XS1100 Newbie]

Just a question i thought i would throw out there for possible answers and thoughts. Twice now i have taken a ride up to Mt St Helens. I am not sure of the exact altitude, but it isn't too high, about maybe 4000 feet or so. Thing is, on both trips i got great mileage. First time i got 50 MPG, and this past weekend it was 47 MPG. My normal mileage is around 38 to 40 MPG. This bike sees to like less oxygen. I did go rather slow on these two trip, so i think that helped, but then again i was climbing a mountain. Just wondering if altitude being a factor on mileage, is there some setting i should be shooting for on my normal running? It really isn't a big difference, just kind of interesting to me.

[Bonz]

Sounds like you aren't riding fast, even though it's uphill, so that will keep fuel consumption down, relatively speaking. Then you gotta come back down, which really helps fuel consumption! As well, I don't know how high the road goes up in terms of elevation, but St. Helens is well over 8,000 feet high, I believe.

I live in the front range of Colorado at 7,200 feet. A main advantage to that is the thinner air=less air resistance, so the bike doesn't work as hard to push through the air, again, relatively speaking compared to lower elevation. Air density at 12,000 feet is about 40% lower than at sea level given standard conditions, so high elevation has an effect on wind resistance.

[ManagerMike]

Hope this helps......carbs work as a result of the 'bernoullie' effect(hope I spelled that right) which is named after the fella that first described fluid dynamics mathematically. There was also a fella by the name of 'venturi' who also arrived at the same conclusions experimentally. In a nutshell the air flowing through the "venturi" in a carbuerator is what draws, or 'sucks' the fuel from the bowls. Less air density=less airflow= less fuel. This is also why if you relocate to a higher altitude long term, you must downsize the jets in any carbuerated IC engines you bring with you, or they will always run a bit too rich.
Electronically fuel injected engines are mostly immune from altitude changes by virtue of a MAP sensor (which measures engine vacuum), or a MAF sensor (which measures the total mass of the air entering the engine) and can adjust fuel delivery accordingly.

[CatatonicBug]

Did you measure fuel mileage before you came back DOWN the mountain? I would imagine the previously mentioned factors, along with the fact that half the trip was powered by gravity would really help the mileage.

[dbeardslee]

I wonder if the lower air density would cause your slides to come up a bit later causing you to run longer on the low circuit and improving your mileage. My understanding of CV carbs is that they're supposed to compensate for atmospheric variations. JAT

[XS1100 Newbie]

That is kind of the idea i had, but i cant see how i can make something to change the air density at sea level. An atmospheric chamber surrounding the air intake?