Your 1/2 tons probably came with 4 1/2" wheels that are designed for 6.00 or 6.50 x 16 tires. Anything wider will cause the sidewalls of the tires to flex too much and generate heat. 15 x 5 1/2 wheels were available for 7.00 or 7.50 x 15 tires were available for a heavy duty option. 45 to 55 mph is what these vehicles were designed to run.