Hi, just had new 18" Contenental DWS tires installed on my '04 G35 Coupe. The Fronts are: 225/45zr18 (same as original tires); the Rears are: 245/40zr18 (original tires were 245/45zr18). What are the concerns/issues, etc. with the present set up? Should I have the rear tires change to 245/45zr18 to maintain the same Aspect Ratio as the fronts? You thoughts are appreaciated. Thank You.

 

5mm shouldn't be a problem.

http://www.miata.net/garage/tirecalc.html

 

The difference between the two sizes won't cause you any problems! I went from a 245/45X18 to 275/35X19 without issues as have most members here on Driver....
Gary