Your RPM at any given speed will be 3.55/3.91 lower. Sheeze.
I know math isn't most of Alabama's strong suit, but really...sheeze.
3.91 is a good backroad gear if you don't plan on going over 50ish. My rod didn't have a speedo and I just multiplied the first 2 digits of the RPM in 3rd by 2. 2500 = 50MPH or so. Worked for me. Same 50 MPH with 3.55 would be 2500 x 3.55 / 3.91 = 2270 RPM. Cam isn't even working at that RPM.