Engine peak efficiency is 2800, but you will have some aero losses, so who knows.
On flat ground, 4000 all day won't hurt anything.I normally run ~3200RPM at 75 so I don't get a big ticket.
If it heats up, slow down a little.
My car will heat up if cruising over about 90 for too long, but we have too much traffic for that here anyway.
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