275 - 40 - 18
 
The middle number is the aspect ratio like you said, the hight of the tire is 40% of the width. 
 
If you run 40 series tires all around but get wider tires for the back they will appear just a little bit taller. 
 
If you run a 35 up front and a 45 our rear, the rear will look HUGE and give you more sidewall. Some guys run a 40 front and 35 rear to make the tires appear even in sidewall height even though the rear tire is wider.