I've used suregrip 3.23 ('68 Road Runner, auto), 3.55 ('73 Road Runner, auto), and 3.91 ('70 Super Bee, 4 speed), and have liked them all for street cars. The 3.91s were a bit steep when I drove it from NE Ohio to Carlisle, but the big L-60-14 Mickey Thompson's helped. If you're going to drive the car alot on the highway and longer trips, I'd go with 3.23's. If you're just driving around town or short trips, then the 3.55's or 3.91's would be fine. Just my opinion. Mine always had the 489 case.