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After careful examination, I determined that my front air dam was not effectively channeling air around these larger front tires I am temporarily running. In fact, I am pretty sure it was channeling air right into the edges of the tire tread. My other “street” tires were about 31.7” in diameter and 6.6” wide, whereas these “all terrain” tires are about 33.2” in diameter and 7.2” in width. (Tread widths not section widths.)
To correct this, since I am unable to easily make the air dam wider, I changed how I was attaching the air dam and spaced it about 2” further forward. Since the air at this part of the vehicle is traveling at an angle, this kicks the air out around the tires roughly the same as if I had made the air dam wider. Per Hucho, moving the air dam closer to the nose should help with front lift anyway.
On my morning commute, I cracked the windows, and couldn’t hear the front tires anymore (over the sound of the rear tires that is) so it must be doing something.
Additionally, when I first swapped the tires I did not air them up to the same PSI as I had been running the other tires, so I also increased the PSI to what I had been running. It is fairly well established that higher PSI will result in better MPG, I am just trying to balance traction and wear into the equation as well.
Between those two things, my trip MPG was showing better than it had been, but I will wait for a full tank to surmise how much of an improvement I got.
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