You can co up a size but that will throw your speedometer out of wack, it will indicate slower than you are actually going. Lets say you are going 70 mph, your actual speed will be closer to 80 mph.
It's not nearly that bad. With tires around of 26" every .10" change in diameter is around 3-4% on the speedometer. For example, if it reads 60 mph with 26.0" tires, the speedometer will read 60.2 mph with 25.9" tires.
