Treadlife rating is molded into sidewall of tire. Take the first two numbers of the rating,multiply by 15,000 and you will have an approximation of treadlife if you RELIGIOUSLY rotate, balance, and check air pressure. Examples:
200=30000 miles (20 x 15,000)
300=45000 miles (30 x 15,000)
340=50000 miles
370=55000 miles
400=60000 miles
You can expect HALF the rated mileage if you don't maintain the tires.
Mark