Originally Posted by
davestv
42/17 for 18.5 mph is a cadence of 96 RPM and a 42/16 for the same speed is 91 RPM. You might just be spinning too fast.
A cadence of 85 RPM will have you going 18.5 mph on a 48/17.
This is assuming you are on a 700Cx23.
I've never measure my cadence but I spin comfortably at 19-20mph on 42/16 and this is with 700x25 tires.