This is the calculation I needed

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/40-16 3.1in 11.1in 22.1in 69.6in 911 0.0%
195/50-16 3.8in 11.8in 23.7in 74.4in 852 6.9%

Which gives me a 7% increase in top speed? Or am I reading it wrong?