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You ask a question, then give your answer that doesn't fit the question. You are adding speed into the equation when you are looking for peak miles per gallon or liters per 100 km or however you measure fuel economy. If it happens at 35 mph, then it happens at 35 mph. If 35 mph isn't allowed on say an interstate, that doesn't mean that your peak efficiency now happens at 55 mph.
To answer the question for my TDI touareg I'd say right about 38 mph in 7th gear which puts me at about 1300 rpm and it goes over 40 mpg on a steady level road, and that's with winter fuel. The problem is, I can't force the shift to 7th gear until I'm goin over 40 mph so I have to get it up to 41, shift to 7th, and then let it drop back down to 38. If I go as low as 35 mph it will probably shift back into 6th with any addition of throttle. It still gets pretty good economy at 55 mph and will run in 8th gear there but it's more like high high 30's mpg vs into the 40's. At 65 it's dropped into low 30's
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