I've been getting the TDR magazine since 1999 and I think there's a total of 4 Dodge Cummins featured with 950,000 miles and above. All but one was still on the original unrebuilt engine.
The highest mileage I've known of personally was a 1991 with a whopping 1,240,000 miles on the clock. I heard about this truck so found out who owned it, a guy named Marty Byrd in Indiana, and I called him up. He told me all about it (used it to transport travel trailers). It was on the original motor and he'd never had the head off the motor. It was still running good although it was using some oil (only a quart every 2000 miles). He changed it every 10,000 miles (about once a month). He replaced most the fuel system at 600,000 (injectors, pump) and was on with 4th or 5th transmission. The greatest weakness was the front suspension, he had to rebuild it like 5 times.
Even at those miles he was still trekking cross-country with it regularly.
I talked to him about 10 months later and he'd sold the truck. He'd put another 20-30k on it (so it was around 1,270,000) when he let it go. A wrecking yard wanted it for the motor, but a farmer in Kentucky bought the truck for around $4500 to pull his horse trailer. Marty had kept the truck up and it was still looking pretty good.
Last time I talked to them they thought the farmer was still driving the truck but they hadn't kept in touch with him. So I don't know what ever became of it. They said Cummins was very interested in the truck but they didn't really offer Marty much $$ to buy it.
I first heard about this truck on the Happy Campers website when it hit 1,000,000 miles, so I posted about it on the Diesel-Stop website in 1999 (scroll down a ways)
http://www.thedieselstop.com/archiv...82&page=19&view=collapsed&sb=5&o=&fpart=1.htm
Vaughn