[slowmover]

In my own case the penalty "average" has been 34%. Done at a higher speed than I now use. The current goal is to hit a higher mpg number that would reduce my fuel burn from 17-cpm to around 14 if I'm remembering the per gallon cost I last calculated. And be just under 30%. From 25-mpg solo to 17.5 towing if I'm at all lucky. A one ton pulling a 35' travel trailer.

To be able to "use" your tests in consideration of a deflector on my bed topper, the elimination of variables, as well as tractive drag problems, would need to be in place. Despite all the other potential problems, I can also look to RDefayettes aerolid on his twin to my truck as a base. From two angles.

This proposed set of tests by you then leaves the door open for further changes. Better tires for FE on truck. Other potential upgrades to same. Upgraded tires on trailer to avoid interply shear problems. Trailer suspension upgrades to decrease tracking problems. And a VPP style hitch to pretty much eliminate tracking difficulties altogether given increased wheel travel and shock absorber installation already done.

A better trailer is the final piece of the puzzle should that ever appeal. Bigfoot, Airstream, Oliver, etc. That these will about last forever is the reason for having one, versus the short one decade life of a conventional square white box. A used trailer of these brands is more than economical.