I'm running the 45mm Mikuni. I love it and so does the bike! At that time I made the switch, I was going to go with the 48, but my wrench at the time told me that he talked to a guy that has a lot of experience with Mikunis, and he said that the biggest difference he has ever seen between the 45 and 48 was 5 horsepower, and that was a 145 ci monster motor! He said the 45 will be much better at day to day riding and more forgiving at low rpm. The 45 is also better at cylinder fill at mid and low rpm than any other carb on the market, which means more torque at the rpms where we ride 90 percent of the time. He also said that there is no ac/carb combo out there that will flow more air than a Mik 45 and a forcewinder, so this is what I went with. Also, at the time you couldn't get a 48 in a polished version, so thats all I needed to hear. It is one of the best upgrades I've done to my bike! I got 46 mpg on a long trip last summer, so it's a great carb! A lot people believe that an s&s g will outflow a Mikuni at WOT. Not true! Once a manifold is added to the flow bench, a 45 and 48 Mikuni both make better numbers than the g carb.