The math says 9.9%.Adding that same % to the tire circumference, in your case is more logical. A 26.4" tire will do the same, and just happens to be a common 15" tire; the 275/50-15. Or a 245/60-15. Or if you really wanna cruise, the 235/75-15 will get you 28.9", a whopping 20% reduction,or so.
