you are barking up the correct tree sir. The first number is the width. I don't understand why people think a 275 x 18 is wider than a 275 x 17. Both are 275 mm wide.

It's the aspect ratio of the sidewall. Overall tire height should be a concern.

275's on a 17 inch rim and an aspect ratio of 40 will yield 25.66 inches height

to maintain a similar height, but on a 18 inch rim and 275 mm width, then the aspect ratio needs to go DOWN to a 35 which keeps the height near 25.6

what the OP will be doing is akin to adding an addtional overdrive gear. His rpm's should drop a couple hundred rpm at highway speeds. It also has a negative affect on gearing. if you have 3.23:1 gears then increasing height makes the gears behave more like a 3.0:1 (i didn't bother with the math on this)

 

245/45/18 is 26.7 diam, 245/40/18 is 25.7 diam, stock 93 size 235/55/16 is 26.2 diam. with a 245/45/18 you are 1/2 inch taller. i am lowered an inch with 26 inch diam tires with no rubbing.

 

Now there are the smartie pants math numbers I was hoping to see...so running a 245/45/18 should have no effect in rubbing and VERY little effect on his speedometer...Thanks Craby you sir are a gentlemen and a scholar

 

Thanks for the tips I went with the 245/45/18's after all. They did great on my test drive around the city no rub on with or height. Note that I still run 245/40/18's on the front. Thanks again!!
http://ghostshipchevelle.com/Home.php

 

Riken Raptors. Nice choice! Not great for traction but will give you a BUTTLOAD of wear.... Client of mine had those on his Mazda3 and got a little more than 100k on the set before replacing them again.

 

the amount the speedo will change is extremely insignifigant,,, were talking 1-2 mph,,, you couldnt fit big enough tires under a camaro without lifting it to change it more than 5 mph