I like the 19 to reduce the drone of constant high speed revs when on the interstate or other wide open roads. The sound of the America with TORs revving through the gears is sweet music, but after getting into 5th gear, I like the lower rpm for cruising. Changing from a 17 tooth to a 19 tooth will give you about a 10.5% reduction in ratio/revs and from a 18 tooth to a 19 tooth is about a 5.15% reduction. In the end, it's a matter of personal preference, as the Americas/Speedys are tough and can handle all available ratio combinations easily. I have a new 17 tooth Triumph brand sprocket for sale in the classifieds that I have reduced the price on and will take reasonable offers on as I won't be going back to the 17/42 setup anymore.
