Going up one tooth will make a difference in gas mileage primarily on the highway. If you ride mostly all highway miles, you'll notice a slight improvement. But around town, I don't think you'll notice any difference. My America came with a 17/42 and I changed to the 18T. I liked the taller gears and not looking for 6th, but did notice a little less oomph starting off two up.
Now that I have the BB installed and way more torque, it has plenty of oomph, and I can't tell the difference when my wife rides.
