As far as tire hieght making a RPM differece at the finish line my Duster crooses the finish line at 7000 RPM at 134.4 MPH with a set of M/T 315x60x15 ET Street radials (94 inch circumference) and crosses at 6500 RPM at 134.6 MPH with a set of M/T 10.5x31x15 W bias plys, very near the same ET also I would definetly try the tires first, maybe a set of radials in that size