We can buy a VW Passat in the States, but not one with a 1.6 liter diesel. US spec Passats have a 2.0 liter gas engine.
It's a conspiracy, I tell you.
Judging by the route and speed, I'm really not surprised.. I'm pretty sure a CRX HF in prime condition or a Civic VX would be just as capable of this kind of mileage under the same conditions. Just remember, the '86 CRX HF got 74mpg highway on the CAFE MPG cycle and the Civic VX got 70-72mpg highway on the same test cycle. I think the only thing special about this test is that the Passat is a much more "comfortable" car than a CRX or a Civic, regardless of trim.
From 85 to 87 I drove two VW diesel pickups for deliveries, the 79 was a 4 speed that got 58 mpg average and the 81 5 speed got 62, hwy and city. Much like a VW bug, you drove them flat out, they were hilariously slow, 18 wheelers would pass me on hills. The only thing slower than me was the VW Vanagon diesel, with the same 1600 diesel in the pickup. I think as a city car, the 1600 diesel would be ok, but you are foot to the floor on the highway.
Now the 80's Turbo Jetta 1600 was a perfect car, the turbo got rid of all the slowness. I think thats why they give you the 2 litre in America.