Originally Posted by noisebeam
Look at the 100rpm speed. Assuming 700x25c tires:
42x16 = 20.6mph
42x15 = 22.0mph
42x14 = 23.5mph
42x13 = 25.4mph
I go with 42x14 if you really are serious about wanting to cruise at this faster speed. It will slow your acceleration out of starts and will be very wearing on headwindy days and long climbs.
Safe bet is just to drop a tooth on the rear cog to 15 get comfortable with it and later go to 14 if you still feel the need. Having a collection of cogs is a nice thing.
Add 50% to above speeds to get an idea of burst speeds.
Another option is to work at cruising at 120rpm with your existing gearing which results in 25mph. Maybe not the most effcient, but very doable.
Al
This is very solid advice. The OP is actually talking about swapping freewheels, but yeah.
25 mph sounds nice, but for city riding fast acceleration will gain you about as much time as a higher top speed and lower gearing is less taxing on your body and drivetrain.