...HAR!!! ....of course that 18" is the inside diameter, Mr. daggo
and probably driving style affects it the most...
still, consider the perhaps minimal effects of the various air-dams, the increasing drag of all the undercarriage pushing thru the murky atmosphere, the frontal resistance as well as drag-suckee-shadow of the Flying Brick plus all that marvelous tubes and hangydowny stuff....as well as gearing etc.
There's a tremendous effect of such as drafting behind-and-in-front-of-large rigs for 20+ miles, which has increased my level-ground mpg indicated to nearly 30 on level ground...but I wouldn't want to do that much, its too demanding as well as a hazard....
I think lifts along affect mpg....but you can't have everything you want all at the same time, as far as I can tell.
MPG on the coast run I made Wednesday gave me 21.69mpg per miles/gal method. Lots of lower speed coastal curves and lots of RV traffic keep MPH down too.
get in--buckle up--hang on
"soothing agricultural implement/personal servant/Walter Mitty multidimensional access utility device conveniently travels on pavement when necessary and often keeps me warm/cool/dry/soothed as needed."