I'm wondering if your gas mileage problem might be because the bikes are sitting at the spot that the airflow from over and under the truck converge, creating a vortex of drag. There have been many studies concerning drag on the trailer/camping website I've been on since '09 (lurking for two years before I signed up)
https://www.tnttt.com/, and I implemented a couple of ideas (one found in an automotive site, as well) while building my trailer, and another, by covering the bed of my tow vehicle (Chevy 2500HD WT). The end result, while towing my 4'x8' trailer behind the 2500HD, there's less than 10% difference in gas mileage, at 75 mph.
Attachment 84262
Seems like you take many
long trips with your bikes. I know that if I were to do the same (not possible, with my knees, I couldn't ride ten miles without wrecking them for weeks), I'd probably need extra gear and supplies.
That's why, on my 100-500 mile trips, towing either my
race car on an open trailer, or my
homebuilt squareback 4'x8' enclosed trailer, I eventually carried so much extra gear that I reinforced both frames, put 3500 lb braked axles under both...
the car hauler got two, and added extra storage boxes on both, as well. But, afterwards, I was glad to have the extra gear along, as were others
at dragstrips where I had extra supplies and generated power, fuel, and air to share, or
much of the same at campsites with my camping group.
Attachment 84261
If, you'd ever decide to sell or replace your aluminum bike carrier, you could easily and cheaply build a light trailer atop a Haulmaster from Harbor Freight $400 steel-framed trailer
https://www.harborfreight.com/1090-l...ility+trailers, or an Ultra-Tow from Northern Tool $600 aluminum-framed trailer
https://www.northerntool.com/product...B&gclsrc=aw.ds
Attachment 84263
i've seen many teardrop trailers (based on the larger 48"x96" HF trailer, and the 60"x96" NT trailers), and they hold together quite well, after a few modifications, as is usual for home-builds); after all, my home-built was a junked older version of nearly the same size (50"x60") as these 40"x48" trailer, and I've semi-abused it over 13 years as a camping trailer (a total of 31 years of ownership, plus ??years, depending on how old it was before I bought it).
The point(s?) is:
- 1) a trailer platform for carrying bikes and/or gear is more versatile (whereas, the bike carrier you have only carries bikes),
- a small trailer will put less strain on the rear suspension (inline with the driveline, and putting less of a leveraged load onto the rear of the vehicle), and
- 3) a trsmall trailer will add less drag to the back of the car/truck (because it follows out of the main vortex, and the bikes are sitting lower to the ground).
Just my honest opinion, as a long-time user of trailers (I have a spare utility trailer in my back 1/4 acre, a 5'x10', just in case I need it...I wish I still had my 18' car-hauler, too!)
And thanks, I believe in illustrating my posts for better understanding (a picture is worth a thousand words, I believe). I use Microsoft Paint for modifying photos and posting them, and MS Photos for enhancing them, or to remove backgrounds (I've never had a professional photo app, like photoshop, because I'm cheap).