All the horsepower estimaters I've ever written and used needed to know the RPM's at 1:1 in the tranny at 60 MPH.
I did Harleys and Chevys when I was doing it. But that was 10 years ago. I don't have a copy of the code, but the forumlars are easy to get.
In fact I calculated the RPM from Gear Ratio and tire diameter!
Maybe we need a new web page? :D