Ok, your speedo will misread by -7.1% and you will also lose quite a bit of acceleration. At 30 mph you will be doing 32 and at 70 mph you will be doing 75. Specification Sidewall Radius Diameter Circumference Revs/Mile Difference 235/60-15 5.6in 13.1in 26.1in 82.0in 773 0.0% 235/70-15 6.5in 14.0in 28.0in 87.8in 722 7.1%