Originally Posted by my bandit
285/40/18 in back
Ah, that explains it then.
Stock 245/50/16 or 275/40/17 is 25.65" and 25.66" tall respectively.
Your tire (285/40/18) is 26.98" tall. That's a big difference.
So with those specs, the calulator at Discount Tire says that if your speedo is showing 75mph, you're actually going 78.89mph.