My new 2013 Outback is rated to pull 2700 lbs. Prior to pulling a trailer I was getting 29.5 MPG measured manually doing math not using the car computer although the car computer agrred. While pulling a trailer weighing approx. 1500 lbs on the interstate I got approx. 17 mpg which I have no problem with. But I have gone through about 11 tanks of gas since and I now get 24.5 mpg every time. I have lost 5 mpg. The dealer insists the computer will relearn over time to get me back to my original MPG. This sounds silly. The sensors in the car know instantly want the load is. Subaru cust. ser. had offered no intelligent input except to say take it to the dealer. Does anyone have any ideas. The car has about 6000 miles on it.
Couple of things you do not have enough miles on your car yet to really be worried about mileage as it will be pretty unpredictable for a while still as the car gets broken in.
I tow a bunch with my subaru and spent 11yrs towing with my prior 2.5 subaru. A few things about towing and mileage. Wind drag is the single largest factor that impacts your mileage when your towing on fairly flat highway. My 21ft racing sailboat which is about 1700lbs with the trailer behind the car - it is very wide 8.5ft wide and sits about 10inches taller than the roof line on the car. When I have any sort of head wind and doing 65-70mph mileage will run in the 16-17mpg range. No head wind and running between 60-65mph I will see upwards of 21-23mpg tank averages.
If you are hauling one of those open utility trailers with the big tip up rear gate/ramp those are the single worst trailers you can possibly tow due to the massive air dam / parachute effect the tall rear ramp creates with the heavy screen type mesh. If you tow long distances at highway speeds I highly suggest you see if you can replace the gate with a short one and find some ramps if you need them. Or even just pop the gate off the trailer when you run long trips and keep a couple of simple ramps handy.
Around town slow towing for yard work crews etc that sort of set up works well but the massive amount of drag they generate at highway speeds will cut your towing mileage nearly in half. So keep that in mind.
My very compact 4x6 12 foot long 980lb tent trailer which is about dead even with the bottom of the rear hatch window regarding height and is narrower than the car - we run between 23-26mpg 65-70mph flat towing - and in the hills 60-65mpg we average around 21-22mpg.
You can't judge your mileage based on the computer if your running the same tank of gas you were towing with prior to leaving the trailer home etc. Meaning you need to zero out the odometer ie fill up etc and run the car without the trailer to get your mileage averages with the car in non towing format.
Every Subaru from my old one to my current one and a couple of friends with the newer 2013 engine - have all seen their averages increase as they get miles on the car. Both of my Subarus didn't start posting solid nice mileage till they had 15,000+ miles on them. Even at 25,000 miles we were still seeing slight gains in mileage.