How bad will bias ply tires decrease gas mileage?

How bad will bias ply tires decrease gas mileage?
The rolling resistance to be overcome regardless of tire type at 55mph requires under 10 horsepower of a typical passenger car. While bias ply tires do offer more resistance, the difference is extremely small - a bias ply may require the maximum of 10 hp, but even if the radial reduces this to the minimum, about 6 hp, the 4 hp difference will take a full tank of gas under exactly the same driving conditions to measure a difference in gas mileage. Gas mileage is not a valid reason, alone, to choose radial tires. However, noise, ride comfort, tire heat, adhesion, are all reasons not to use bias ply unless really necessary. In addition, Nylon bias ply tires "set" in cold weather and result in a "thump-thump" until they warm up.