I run 39/15 on my Schwinn (the 39 was the stock inner chainring on the 1pc crank) and I spin out around 25-26mph, which I'm getting a little tired of doing on the flats where I want to go faster. With my 27" wheels this is about 70.2 gear inches or something. At 70 gear inches, I can keep 19-20 mph without too much of a problem, 21-22mph is spinning a little fast but I'm getting comfy with that. I was going to swap in a new chainring, probably a 42 for now, but I'm going to fixify my old Fuji soon, and will probably run a ratio that gives me gear inches in the high 70's or around 80 so I have a sporty bike with higher gear inches and a workhorse with lower gear inches (probably throw a rear rack on my Schwinn, etc).