Tire Sidewall problems of the 1997 Nissan Sentra

Failure at 40,000 miles on 80,000 mile goodyear aquatread replacement tires not original equipment 1normal driving. Months before hit speed bump at 20 mph. Another time vehicle was overloaded by 200 lbs for 7 miles. 2 stess cracks on side wall of one of the aquatread tires, slow loss of air preasure in tire. 3 replaced all four tires at recomendation of manager at goodyear tire he explained he had never seen stress cracks on the side wall of a passenger vehicle tire in his 20 years experience. He had only seen them on truck tires . He thought it may be a manufacturers defect and sent the tires to goodyear for xray analysis. He also said its possible that a speed bump or overloading could cause this problem.