Are you spinning the tires or the clutch ? I mean I have 4.30's with 30" tall radial slicks and I can be going 60 mph at 3300 rpm and hit it hard and it will easily run up to 120 mph before I crank over 6500 rpm. But I drive on slicks so the tires usually stay hoooked when I step on it at 60 mph. And by the calculations my eng makes about 600 flywheel hp. Just seems like with 3.91's that wont happen so easy unless something slips. Ron