[slowmover]

Absolute numbers are of interest, but the percentage change is what counts when talking about RV fuel economy. The fuel burn penalty range is at least 30% for most vehicles, and 40% probably the norm. Over forty is, IMO, poor driving skill as well as poor combined vehicle lash-up ( as well as steering slop, bad tire pressure, worn suspensions, etc). Under thirty is more likely evidence of a motivated driver. I've been postulating that an EM reader can, with the right set up, keep losses under 30%.

The test set ought to be a distance covered while on cruise control where:

1] Solo, open bed

2] Towing, without Aerolid

3] Towing, with Aerolid

4] Towing, with Aerolid configured for towing.

First, the question by other RV'ers will be how are the pair of vehicles loaded? For the TV it will need to be with full fuel, regular gear aboard, and loaded for travel (inclusive of passengers, etc). Same for the TT: full fresh water, full propane, standard gear aboard and then loaded for a trip. In each case, weight representative by sand bags, etc can be done (with a pass across the scales on a regular vacation trip to verify).

The published base weight for your vehicle as against the actual base weight of driver, full fuel and standard gear aboard. The true solo weight. Then the solo weight as for travel. Same for trailer. An initial trip to the scales for adjusted (true) base weight, short of passengers and per trip extras in both vehicles.

Second, it will be brought up the in [1 & 2] that an open bed can be modified by a partial tonneau that covers the last 50-60% of the bed so as to equalize pressure against both sides of the tailgate. A piece of plywood would work. This is against what one would assume you already have as solo highway travel in your records as an average (same speed as when towing, on cruise control).

I would want to use an Interstate for the test course so, again, to keep results most consistent with one another. And I would think a distance of 100-miles both outbound and inbound, but leave that point alone otherwise.

Second, a default setting for the weight distribution hitch needs to be set. 100% FALR would be my suggestion (where the TV front axle is returned to its solo weight value), or one can work with more current, but contentious OEM values of 50% plus. For more on this, see article by Andrew Thomson in his column, Hitch Hints, in Vol. 44 #1 of RV Lifestyles magazine, 2015 Buyers Guide, Nov. 2014, Page Six. (Can't figure how to set link with phone).

As well, a trailer axle alignment plus brakes and bearing preset verified as well as trailer tires to full sidewall pressure. As in other posts recommending this avenue of pursuit to bring steering corrections to their lowest point and for best braking/handling performance, equalizing the axle pair via loading as seen on a scale per wheel position as well as each axle is to the point of repeatable results. A trailer even slightly out of alignment and with unequal loads cross axle and per axle will track differently in changed winds (outbound and inbound on the test course). Skewing your results for no good reason past poor preparation.

That it will reduce tractive drag is all to the point of the testing. Same for truck, but where the alignment is verified, brake caliper drag is not apparent, and tire pressures are within OEM door placard spec.

The percentage change from solo to towing, over the possible range of changes, is then closest to accurate. Or, can be repeated by others where combined rig variables are minimized.

Also needing note are:

A) Climate

B) Terrain

To round out variables for comparisons by others.

The point would be, long term, to be able to predict average fuel cost on a per mile basis. Cents per mile (as is used in business calculations).

I know I keep pushing these things, but there are plenty of savvy RV'ers who would be interested in the results. Not covering the above leaves, then, too many ways to ignore results you've already spent time and money upon. Being just slightly painstaking about WDH set up and lowering tractive drag will obtain results consistent enough to silence skeptics. That it is neither a fluke or just a stunt. Not "real world".

I'll put it another way. Give me your rig and let me correct problems, optimize, and I'll show a better percentage change than with A-lid alone. The real problem for combined rigs is in wind handling, not aerodynamic drag (as that is a function of design). The real world has crosswinds and bad road surfaces to deal with. Average towing mpg takes its hits ( as well as safe traveling challenges) from these.

Tracking percentage mpg change is the key. Optimal baseline is in correcting all mechanical deficiencies first.

.