Here is a circumference calculator on discounttire.com. You might want to try to verify how accurate your odometer is overall. My CRX has 185/60/14s on it which should be about 4% larger in circimference then the stock 13 tires. I used Google Earth to measure about a 30 mile section on my commute to try to see if I was really getting 4% lower mpg then I was calculating, and it turned out the odometer was pretty much dead on with the 14" tires. I guess the stock odeometer is about 4% high.
Tire calculator says 165/70/13 has a circumference of 69.41 and 175/70/13 is 71.14. This is 1.73" more. 1.73/69.41 is 0.0249 or 2.49% more. The Sunday night football game must be pretty boring to make me want to calculate tire circumferences.