okay,17/37 gives you roughly 2.17:1 ratio, 17/34 gives you 2.00:1. you could change the front sprocket as well. the other variables are the gearing of the transmission and rear wheel height.
so, if at 70 mph, you're at XXXXrpm with the 17/34 setup, then you can calculate the difference for what you would be at with the 17/37 setup.
..sorry, i started rambling. (reminds me of manual transmission class in college)