10.5Wx31, 13.5lbs. Car weights 3340-3440 depending on ballast. 1.25-1.30 60 ft. 10.5Wx28, 15lbs. Car weighs 3890. 1.53-1.55 60ft. Sounds like your rebound settings are too loose. We've had good luck with M/T radials/ Afcos. However they have been revalved stiffer by Menser Shocks. The heavier the car, the smaller the slick, the more air you need.
Doug