[jmo]

should I take the Topper off to gain some capacity OR does the aerodynamics of the topper when towing make it a non-issue OR OR.. just not worry about it!

When towing my boat with my old 2001 Tundra that had a leer cap on it I found it helped with fuel economy because the slipstream coming off the back of the rig goes right over the windshield of the boat (I know since the boat cover isn't being ruffled or put under tension from the wind).

With my 2006 that does not have a cap on it I can the front of the boat is inducing more drag given the cover is definitely getting pummeled by the wind.

I'm pretty confident the cap for my towing application was giving back at least 1 mpg, possibly a little more.