I towed my Nissan Versa (when i had it) from GA to TX on a car dolly from uhaul. I got 10.5-11 mpg. (That was according to my trip computer which hasn't been corrected for my tires yet. So it may have been 12. While those all seem bad my xk doesn't get much better anyway. Maybe 13-15 at the most. I've found that no matter what i tow it seems to always be about the same 10.5-11.