Well, I'm running 3.91's right now in my Roadrunner with 26-1/2" tall rear tires. The suburb highways are 70 mph & I'm running right at 4000-4100 rpm in the slow lane...… my well-built, balanced, all forged motor sounds like it's going to spit a connecting rod out the side of the block on the highway. I personally wouldn't recommend 3.91's for the street.

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"