Can you replace 205 55 r16 with 225 40 r18?

205 x 55% = 112.75 = sidewall height

112.75 x 2 = 225.5 = 8.87795" + 16" (rim) = 24.87795 diameter

225 x 40% = 90 = sidewall height

90 x 2 = 180 = 7.08661" + 18" (rim) = 25.08661 diameter

As you can see, the diameter will be only 0.20866" larger, so I would say YES !