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Since there is no mention of a budget, I suggest a new BMW M5. These will comfortably eat your 500km each way, and you'll always arrive in comfort and style. Great for the family too.

For economy, the Audi A8 TDi V8 gets 7.6 l/100km, and will accelerate 0-100 in 5.5 sec. These would also be a great long distance cruiser. They are, afterall built for the autobahns.

However, if you're not an Audi or BMW fan, then you'll just have to settle for the Merc S63 AMG. A bit thirsty, but will accelerate to 100 in less than 5 sec, so safe for overtaking those road trains out west.

Hope this info has helped you, Adam.

can someone explain why a large car is more fuel efficient on highways/long distance trips?

They aren't necessarily more efficient, but there's far less difference between a big and small engine when a big engine is just loping along at a constant speed well within it's ability. The torque helps, too. More go at lower RPM means lower overall gearing and lower revs as cruise speed which equates to lower fuel usage.

yeah it's a mix of more torque, taller gearing, inertia and general physics. most large cars are generally detuned quite as they are designed to be good at highway cruising (hence why plenty of falcadores will get better economy on the highway than a more highly strung natro skyline). and the fact that the amount of extra power required to hold an extra few hundred kgs at 100kmh isn't that much. so not much extra fuel is required.

the best economy i've gotten out of the missus SSS was 6.5L/100kms, but after driving it to work the other day, it reminded me just how much more comfy the seats are in the commodore. and the general driving position for that matter. not as nice/fun to drive, but much more comfy

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