yep, that's almost correct you'd actually be going 1.3% faster than your speedometer says (cuz the 235 has a bigger circumference than the 225 so each revolution you're going a bit further). so if your speedo says 60, you're actually going 60.8.
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
205/50-17 4.0in 12.5in 25.1in 78.8in 804 0.0%
235/40-18 3.7in 12.7in 25.4in 79.8in 794 1.3%