Since the OP’s question relates to two variables (gearing and altitude) he might be interested in roughly calculating jetting changes (which roughly translate into power losses) by doing a desk exercise on this site intended for airplane guys. In other words, he is thinking about gearing way up to a 24 tooth c/s sprocket while losing horsepower to increased altitude.
I used this site for years roadracing with AHRMA, where we raced at elevations from sea level and 60 degrees F in Daytona to Miller, at 4500 feet and 85 degrees F, and most relevant for the OP, we raced at Willow Springs, in the Antelope Valley at say, 80 degrees. Needless to say, getting jetting even in the ballpark in that range of temps and altitudes was always a challenge and this site really helped get it in the ballpark. I used Mikuni carburetors, which number their jets in very rough proportion to their flow, so a percentage change in air very roughly translated to a change in main jet which made jetting off this calculator conceptually easy.
You can get real numbers from the weather service. But you can calculate percentage changes for comparison purposes using plug numbers—100% air is about sea level, 30.0 barometer, and 60 degrees F, and the calculation is not very sensitive to humidity, dew point or barometer, so for a rough desktop calculation of how much power you’ll lose to elevation you can use 30.0 for barometer in both parts of the comparison, sea level and 2500 feet for elevation, and 60 degrees and expected Antelope Valley temperatures (80?) for when you will run, and knock off say 30% of the temperatures for dew point just to plug something in.
So that’s a lot of words but cutting to the chase what you’ll find (if my recollection of my calculations for Willow Springs is right ) is that you’ll be running through the traps with 92% air— so you might do something like jetting down that much to be in the ballpark, and expect something akin to that much power loss, for comparison purposes only. With, let’s say, something like 92% of the power as you would have at sea level, or something like just a bit less than 10% power loss you’d lose something like 4 hp on a 50 horse engine — certainly nothing precise here but it gives you an idea of power loss to altitude—and you are proposing gearing the thing up 20%, with less power. I’ve never raced my street 850 Commando, but my impression is it’s gearing is maybe a bit short with a 20 tooth c/s and it might be able to pull maybe one more tooth on the c/s at sea level but possibly not with 8% less power. Anyway, this is just another thing to consider to rough in an answer to your gearing question for running at altitude.
By the way, this site now contains a link to a similar calculator for engine tuners
https://wahiduddin.net/calc/calc_da...m above it calculates a 14% power loss, FWIW.
