For those people not used to the language, PM stands for preventive maintenance. Each marked car gets either and A or B set. Bothe include and oil change and check up, B includes a brake job. It is set for every 4000 miles. On an average when cars were run hot on all 3 shifts the car was in the shop every 10 days for maintenance and not available for service. I go about 3 months in between tune-ups. Ad in the factor of 3 different people with 3 different driving/braking techniques. This will wear