Shop: Halter's Cycles
I think the power of the engine has little impact to MPG when performing the same amount of work. The drag to push through the air is the same regardless of your engine power. You can see this by looking how close the MPG on large SUV's are regardless of the engine. I used to get roughly the same MPG in my 5.3L Envoy as a buddy got in his six cylinder version of the same truck. Engines were doing the same amount of work so used the same amount of fuel on the highway.I am actually most surprised with their results with tire air pressure.
I also think that the effects a roof rack/box has on your car has a lot to do with the horse power of the engine. The smaller the engine the more it has to work to cut through the air, and thus the greater effect on your fuel economy. My old civic with it's tiny 115hp engine would drop from 36 mpg to 31ish with the rack on.
Tony, what year is your Accord? I can't break 26 on mineI take the roof rack off my car during winter months mostly because I don't like to keep it on my car all year round. I just took them off this past weekend and I'm now using a trunk mounted rack as I don't ride as often in the colder months. The Thule trunk mounted rack is pretty easy to put on and take off when I need it.
As for fuel economy, my car's trip computer has been registering 30.5 to 33.5 mpg on my Accord with the roof rack on the car. I'll see after a few tankfuls if I notice any significant increase now that the roof rack is off.