Is the B6 a vacuum activated fuel tap like the B4?
If so, probally even though you turned it to 'reserve' fuel wasn't getting to the carbs and filling them. You can just about crank the battery dead before the engine will generate enough vacuum to fill the carbs. I had the same thing happen on my B4, cruising down the expressway at 80mph and started loosing speed. I managed to coast off an exit ramp and pull over. An inspection of the fuel line showed it was bone dry (which was good, because at first I thought something had broke!). Turned it to prime until the line filled up, flipped to reserve, and rode another 10 miles to a gas station.
According to what I found in the internet, the fuel tank of the B6 is 5.3 gallons. I'll assume that there is a 1 gallon reserve (I'm assuming you still had reserve gas, and just didn't prime the carbs), meaning you burned 4.3g or so (guessing). 196/4.3 = 45.5mpg. That sounds about right, I'd think!