Here is my experience with the 585 cam and fuel mileage. Now you have to understand that I have a mikuni. You can taylor the fuel a little more precisely to what the engine wants. You can change the needle to a leaner 98 needle. Stock is 97. Changing the needle only leans the mixture in the 5-25% throttle range. This is where all of your cruising is done. I tried the 98 needle with the stock cam. The bike ran ok, but you could just tell that it was too lean. Throttle response under 25% throttle was sluggish and maintaining speed on a hill required a bigger throttle opening, which uses more fuel. I never got any better fuel mileage with the 98 needle. Sometimes it was worse. I went back to the 97. With the stock cam, I was getting around 40-42 mpg. I could get it up to around 45 if it was all highway miles. When I installed the 585, I lost 3-4 mpg. This wasn't unexpected, but I still wondered if I could do anything about it. I figured I'll try the 98 needle again. I wasn't expecting much because I remember how it was before. The 98 needle was much better. The throttle was more responsive under 25% and fuel mileage improved dramatically. I'm getting at least as good mpg as the stock cam. Sometimes better. We went to the Flight 93 Memorial two years ago. Coming home, I checked my fuel mileage. I got 47 mpg on that trip. I'm not sure why the 585 likes the leaner 98 needle. It's seems counterintuitive. Why would a performance cam want less fuel at lower rpms? I don't know, but I have a theory. Performance cams have much more overlap. That is when both valves are open between the exhaust stroke and intake stroke. At high rpm, a lot of overlap helps get more air and fuel into the cylinders. At low rpm, it actually causes positive pressure in the intake. The piston is moving up when the intake valve opens. This is what causes the well-known "slide rattle" when using a performance cam with a mikuni. The pressure changes in the intake push the slide in and out causing a clicking sound. Overlap also causes the chugging sound at idle that everyone loves. I believe these pressure changes at low rpm cause the mixture to become rich. I believe that's why the 98 needle works better with the 585. Now, the equivalent of changing the needle in an S&S carb is going to a smaller intermediate jet, but it has more duties to perform than the needle does in the mikuni. That may not be feasible. You might just have to live with the loss in fuel mileage, but it is a really nice performance gain.
