"I got better gas milage, even loaded down for 3 week trip, when I have the 18 ft canoe on top."
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I can certainly beleive that on a sqaure ended vehicle.� Our 95 Escort wagon with a long slender roof top box on that extended to the very rear of the vehicle resulted in several tanks on mid 40's (!)�mpg on a x-country trip.� I had at least 3 tanks that caluclated over 45+ mpg, one at 47.� No, not the same pump each fill-up obviously, but the tank before and after were not abnormanlly low, so they were pretty accurate.� 
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Like Don, I always keep tabs on my gas mileage on every tank to track if anything is wrong with the vehicle.� With my daily driver Civic 4-door, I use a ScanGage2 - great device.� Even bare roof racks on the Civic cause a huge drop in gas mileage.� The gas mileage drop from keeping those racks on a typical car all the time, as some do,�can easily exceed the price of the racks over the course of ownership.� 
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John
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