[Gschuld]

OK some more data. I used my 24ft, dual axle, 7000lb capacity trailer, strapped down 20 standard cement blocks, and weighted it at my local recycling center. The trailer has a 6’ wide welded steel frame with 2x12 pressure treated deck boards with 1” gap between them all, 8’ width at the tires (205/75/15 Loadstars). Bottom surface of trailer is open steel channel right up to the underside of the deck planks(draggy). The concrete blocks were strapped down 1 block high in a 5 wide, 4 long pattern. Total trailer weight is 2735 lbs.

Jeep Grand Cherokee SRT is 5400 lbs with me in it. So 8135 lbs in total with trailer. The highway had near zero traffic, rolling modest grade of 75ft altitude difference tops. All tests were 10 minutes duration or longer.


MPH. W/Tr No/Tr % drop

55. 23.2. 31.1 25.4
60. 21.3. 27.1. 21.4
65. 20.3. 26.4. 23.1
70. 20.5. 24.9. 20.5
75. 19.6. 21.9. 10.5
80. 18.2. 22.5. 22.5
85. 16.2. 21.3 21.3

Drafting efforts with trailer attached(percentage was based on nearest no trailer numbers.

69. 20.7. 24.9. 16.9. ACC off, kept 50ft distance, semi
69. 19.5. 24.9. 21.7. ACC on, 75ft distance from semi
66. 20.5. 26.4. 22.3. ACC on, 75ft from truck with 10ft tall stuff
62. 21.6. 27.1. 20.3. ACC on, 75ft from LG box truck

So my take is the only clear anomaly is the trailerless number at 75mph. That is a bit off the trend and is likely an error on my part(possibly not resetting the MPG when changing speed from previous test).

Also, drafting behind a semi, big box trailer, etc had near zero effect when pulling this combo. I suspect the trailer was at the convergence of the wake of the semi/box truck and neutralized any benefit I was getting without a trailer in previous tests. There was a clear and repeatable improvement in mpg in previous test when drafting at 75ft (ACC on) with no trailer, from 60-75mph. It got slightly worse mpg actually with the trailer drafting at 75ft. Even taking the ACC (adaptive cruise control) off and carefully drafting to 50ft distance from 75ft, had little effect and improved mpg only marginally.

So, as a whole, assuming I can produce a trailer with a SUV/trailer combo with a bit lower drag than that big open trailer (I’d like to think so), I’d be pleased to improve on the numbers above by a small margin.

At 75mph, 19.6mpg currently with a 2735lb open trailer. If I could break 20mpg average at 75mph pulling 2700lbs of trailer I’d be thrilled. Assuming the no trailer mpg was really 23mpg, 20.5mpg would represent an 11% loss from running the same 75mph without the trailer. Tough, but plausible maybe?

So far it’s all pretty positive news from where I’m standing. And happily inline with what I was theorizing (guessing) was possible.

George