I see 215/40/18 but also 225/40/18 or 225/45/18

I was gonna do 215 /40/18

But wanna know why people run 225

 

Honestly? NO idea but some people may be interested in different width due to how the tire looks on the wheel. As long as the height is not being modified, no issues with speedometer or excessive wear. Not 18's, but I am running the 19 inch TRD wheels with 225's instead of the 235 (Bought the set with the wrong tires to begin with, decided to keep these until I need to buy a new set).

Hope this helps.

 

There is so little difference between a 215/40/18, which is 0.1" shorter and 0.31" less in circumference than the stock 205/55/R16, and a 225/40/18, which is 0.2" taller and 0.63" more in circumference than the stock tire, that it's almost a 'wash'.

My guess is a lot of it comes down to brand, price and availability as why one size is seemingly more popular than another. Ironically, the 215 size seems to be between $2-$7 MORE expensive than the same tire in a 225 size. That's probably why...

 

A lot will depend on how wide your wheel is and what look/clearances you have. I ran 18s I had 225/40s on an 18x8 +40 wheel for awhile and it was great. Eventually when I went to air I would rub at full drop and didn't want to roll my fenders so I went with a 215/40, it was just enough to clear all the way down. Definitely felt the road a bit more due to slightly less sidewall but more due to the stretch on the tire, where the 225s were a square setup. A lot will depend on what look you want and what wheels you have.

In the shots below the first shot is with the 215/40s while the second is with the 225/40s.