What are others experience with MPG while towing? I pulled a 3000# boat up to the mountains and back with two bikes on the roof rack and got about 14 MPG for the 250 mile trip.

 

Not only were you towing, the bikes on the roof were a significant aero drag.

 

Below is a post from the GL section of a member who also tows with a CDI. Your mileage could have been different due to steeper terrain, trailer condition, boat size, average speed etc. 14 MPG does seem a little on the low side. Positive about your calculations?

"This weekend I got 10.2 l/100km or 28 mpg imp. or 23mpg US:
Pulling a 2,500lb trailer through the mountains, then through surface streets cross-town. Average speed: 91km/h (56 mph)
Met another GL CDI owner in the same campground, and he was pulling a similar trailer, but he came down from Edmonton and only got 12.2 l/100km or 23 mpg imp. or 19 mpg US, but his average speed was 130 km/h or 81 mph.
Gotta love that!"

 

239 miles
Average speed 37MPH
14.5 MPG

Ski Nautique and dual axle trailer weigh ~3000 pounds and probably a bit more aerodynamic than a box or camper trailer.

Without the trailer but with the bike racks I get about 20+ on the same trip up to the cabin and back. I start at about sea level and top out at about 6000 feet and the climbing starts during the last 45 miles. Typically I am seeing between 22.5 and 24.5 average on a tankful.

I was a bit surprised at the consumption, but it sure pulled strong no matter the grade or elevation. Oh well, still better MPG than the full size Bronco that used to pull the boat.

 

Ah ha.. the 6000 foot elevation is the clincher. Be glad its turbocharged... Be happy with 14 MPG under those conditions.

 

No, mileage is better at higher altitude.

His problem is the bicycles on the roof.

I towed motorcycle trailer from Albuquerque to Corpus Christi and mileage started at 21 and ended at 19. Coming home, it increased as we gained altitude.

It's all about aero. Moreso than weight.

 

I tow a 7x14 enclosed trailer with various weight loads and get an average of 19mpg. I also tow a 20' 4000 lbs boat occasionally and get a bit less with that at 17mpg. Most of my towing happens in relatively flat areas around the coast. Unloaded on the highway my mpg runs a steady 27mpg just for comparison.

 

Turbocharges must pump more air as the air density decreases in order to maintain manifold pressure (boost level). As more air volume is forced in to the motor, to meet the ECU mixture requirements, more fuel will be injected so volumetric efficiency will be optimized. More fuel = less MPG.
The benefit is that you get close to sea level power at 6000 feet, so one can effortlessly tow that trailer (and roof bicycles) to the top of the mountain. Altitude, weight and speed (RPM's and MPH) are your controllable primary factors. (Not in order).
But if you believe aerodynamics is the way to go, try folding in your outside mirrors.

 

The MB turbo is electronically adjustable and therefore the engine has little knowledge what altitude it's at--you can take to the bank the engine is not running richer (nor leaner) at altitude. You've basically said it yourself--the system is making the engine think it's at sea level and consequently the amount of fuel remains constant.

The variable in the equation is aero drag and since it's less at altitude less work--and consequently less fuel--is required.

 

You can't do that while driving very fast. I always did it on my old ML230 when we pushed it to 119 MPH, always fold the mirrors