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So my gas mileage will go UP if I am tailgated by an semi or SUV?
I always thought it was the opposite. The car in front has the added load of the drag on the vehicle in its wake...right ?
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Most of the drag when you ride down the highway comes from the pressure differential between the front of the car (high pressure area) and the back (low pressure wake). If you reduce the pressure in the front, or rise the pressure in the back, you reduce drag.
So when two vehicles travel together in a convoy, the front one gets its low pressure wake filled a bit the the high frontal pressure of the trailing vehicle. While the drafting vehicle gets some front pressure alleviated from the low pressure wake of the front vehicle.
Typically, the drafting vehicle profits more, though you'd be surprised how the leading vehicle profits not much less (given both vehicles are the same). If you are being tailgated by a big rig, you will definitely see it on your instrumentation. I once had a big rig so close to me I could only see his grill in my rear view mirror. I was doing 50 mph and 70-75 mpg on the Scangauge and when that semi pushed in close, the numbers went up to 95-105 mpg.