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Our Volvos used to do 250k kms a year, double shifted. I'm not surprised Iain's Merc hasn't been any trouble - have you taken it in for its first service yet?
My first truck, an American Mack, had done 1.2 million miles when I sold it to its 3rd owner. The first owner had used it on middle east runs, not exactly an easy life but then again it wasn't trouble free in my ownership. Had to have the block stitched after a crack appeared so replaced internals while we were at it. Transmission was never a problem though, engine, box and back axle were all of Mack's own manufacture whereas the competition (Peterbilt, Kenworth, White) used whatever the customer preferred from Cummins, Detroit, Eaton, Fuller and all were capable of similar mileages. Low revving engines and constant mesh boxes were the key as well as proper drivers.