My Outback is the 4 cylinder model, 2017. On my current ride I am also running Yokohama Geolandar tires which are heavier and larger than stock (size 235/65/17) and also had a Thule Pulse cargo box M above.
This observation may not be useful for those on other models or those with 6 cylinders.
In summary, the Outback isn’t wild about speed. Miles per gallon drops off significantly above 65 mph. This is really not surprising, as wind resistance increases significantly with increasing speed.
However, I was surprised to find a greater than 2 mpg difference between 65 mph and 67 mph.
At 67 mph, I only averaged 28.1 mph for about 200+ miles on I-80 through Nevada (using cruise control and with the AC on). When I slowed down to 65 mph, the mileage started to increase immediately and averaged about 31 mpg (and that’s with a cargo box on top) for the remaining 150 miles I did on I-80.
I then pushed my average mileage up to 33 mpg while traveling along Highway 93 between Wells, NV and Twin Falls, ID at a speed of 62 mph. I then averaged 32-33 mpg the rest of the way to Montana, despite constant speed changes (with a top speed of 65 mph), passing through towns, up and over mountains, and multiple start/stops and some exit off the road.
In short, if you want the best mileage out of your Outback…slow down! If you go over 70 mph, expect bad mileage. The Outback is sleek enough, but it’s not a race car either.
