Tire pressure and mileage

Even our county police have been running nitrogen for highway patrol. May give you some benefit but probably not worth the cost. Now, helium is the way to go if you really want to reduce unsprung weight.

Seriously, pumping your tires up will not only improve mileage, but handling as well. When I used to autocross a Corvair I'd run 50 psi rear and 30 psi front, with much wider tires on the rear as well due to the extreme weight imbalance. For a street Subaru 35 psi, or whatever the tire manufacturer's maximum is, should optimize gas mileage, tire life and handling. And check that air filter. I belive the Subaru interval is usually 20,000 miles, but If you've got more than 10,000 and it's underperforming, it's a good place to start with a small investment.

I have a really good German dial gauge that adjusts for atmospheric pressure.

ROTFLMAO :-D