I like to keep my tires at 40 PSI. When you drive HWY the temperature of the tires will cause the pressure to increase. So they might get to 42 PSI after a long drive. I typically drive 75-85 MPH on interstates.
We are doing another 7,000 mile 3 week run. Recommended 36, I run 38 PSI. A compromise for ride, load and mileage.
We alternate our trips each year between going to Utah + Colorado (3k miles) and going to North Carolina (5-6k miles). We're running 38 front, 36 rear on our trips at 70-80 mph speeds. We originally bumped them up to 40 but the ride was a little too stiff for our liking. We haven't seen any real change in mpg lowering it back down a few psi.
What is your definition of best? If you want to maximize tread life and enhance fuel economy, use 42/40 psi. This will promote even tire tread wear. If you want a softer ride, use mfr spec of 35/33 psi. This will usually result in faster tire shoulder wear.
At highway speeds, I am thinking wind resistance takes over from tire pressure (rolling resistance). So I am starting to think high tire pressure helps more on the driving we do (around town slow).
