Tire Size Sidewall Diameter Circumference Rev/Mile Difference
225/55-16 4.9 in. 25.74 in. 80.88 in. 783 0.0% OE 16"
225/45-18 4.0 in. 25.97 in. 81.59 in. 777 0.9% OE 18"
225/40-18 3.5 in. 25.09 in. 78.81 in. 804 -2.5%
245/40-18 3.9 in. 25.72 in. 80.79 in. 784 -0.1%
255/40-18 4.0 in. 26.03 in. 81.78 in. 775 1.1%

Anything within 1% should be fine, at 50mph your speedometer would be 1/2mph off.

 

I post this all the time but follow the link below:

http://www.wheelsmaster.com/rt_specs...&text1=&text2=

Put your stock wheels/tires in the left and your new wheels/tires in the right. There is a row in the table that is generated below that computes the change in your speedometer.

 

I've had to deal with this on all my previous cars. You won't notice a difference as long as you keep the total diameter of the tire the same.