Buy 3 Tires Get 1 Free!

What am I dumb? Am I supposed to believe that I actually get a "free" tire? Don't you suppose that maybe the cost of the fourth tire is "built-in" to the price of the other three? Yet I see this advertising ploy all the time! Who only buys three tires?

"Yeah, I don't drive on the left rear tire very much so I just need the other three..."

This reminds me of the coupon that Rainbow Foods had last week for 50 cents off a dozen eggs! Cub Foods (both are owned by SuperValu) had eggs for $1.99/dozen and Rainbow upped the price to $2.49 with a -50 cent coupon for a final cost of $1.99! Wow! I "saved" 50 cents!!! NOT!