On my 2003 Ford F-250, that weighs over seven thousand pounds, I just recently replaced the front brake pads at 185,000 miles. Did not turn the rotors. They were the original OEM pads from when it was new.You must not stop very much, or have hills.I replaced the pads & rotors on 2000 Blazer at 115K and again at 155K, I thought 40K was reasonable.I'd guestimate that about 80% of the miles are strictly on long, open highway. And, although the truck has an automatic transmission, it has a "towing" mode that keeps the torque converter locked to allow for some mild engine braking.I think 40K miles is reasonable for the average street pads, but I'm surprised you needed rotors that soon. Even on my car the rotors last for about three pad changes.