Well I got almost 400 miles to do this weekend, but only about 20% is highway. Still, I'll have a go with cruising speeds and see what happens. Unfortunately, there are a few inches of snow here and there, so I might have to slow it down a bit!
I have another question as regards to tail winds and head winds. If there is a strong tail wind, should I decrease my cruising speed and let the wind help push the car, or increase it? If theres a strong headwind, should I increase or decrease or will neither make a difference?
A tailwind lets you cruise a bit faster if you so choose at the same fuel economy. I do things a bit differently - I simply pick one steady throttle setting and this, with a tailwind, lets me run a bit faster with better economy. The opposite is true of headwinds.
Although by going slower it makes it longer that you travel into the wind and the more it has time to exert force against you however since drag increases with the square of the velocity it is best to not go too fast... Tail winds work great when you drive a brick like my 2006 xB but going faster with a tail wind increases pumping losses due to light load so it sort of cancels the tail wind a bit if you go too fast. It is fun going 70-80 mph and get 50+ mpg though!