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High speed can absolutely murder your fuel economy.
I suggest picking a speed and sticking to it regardless of higher speed limits.
What matters for travel time is average speed, not topspeed.
80 mph is just under 130 km/h, a TDi might get acceptable fuel economy at that speed.
But beyond that it starts getting worse exponentialy.
I suggest 110-120 km/h maximum.
Worst I've ever gotten was 35L/100 km (8 mpg) in an Audi TT with 180 PS 1.8 TFSI at 250 km/h (156 mph)
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