It can handle 120 km/h without problems, as long as the engine is properly warmed up and properly maintained (valve adjusted, regular oil & filter changes, diesel injection system in good order). It is designed to go long distances at high speed. It is a question of preference, you also might want to consider oil and fuel consumption, they tend to increase at higher speeds. Of course piston speed increases, which puts more stress on them, but again, the engine has been designed to go at 4000-4500 rpm.
Maximum speed is about 138 km/h for an automatic. The engine is at 4500 rpm at that speed. If you push it, it can go a bit faster, maybe up to 5000 rpm, but I wouldn't recommend that, not for prolonged periods.
Don't forget the speedometer is usually a bit optimistic, check the speedometer with a GPS thingy.
It helps if you push the engine to its limits once in a while, it burns away carbon deposits that build up when you just go at low speeds all the time.
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