I have 18 X 8 rims on my car. The tires are 245 X 40. Tire rack suggests 235 x 40. Which one is better and which has more traction?

 

All things being equal, the 245 should have a larger contact patch - thus more traction. Of course, all things are seldom equal, so check the traction rating and reviews about each tire first. Some manufacturers excel in one tire size, but the next size up in the same tire just doesn't ride or grip the same. Don't ask me why. Same goes for speed ratings (most assume that higher speed ratings give better grip - not true). I had 2 pairs of Turanza's on my previous car and the H rated tires gripped and handled better than the V's, yet the Z's were supposed to be the cat's meow (couldn't afford those)! Go figure.