When I purchased my SAAB 9-5 it had 32psi in the tires. The sticker on the drivers side doorframe show three different setups with two different tire inflations. I kept it at 32psi for some time, that being the recommended tire pressure for 1-3 people in the car.
The other two recommendations, 4-5 people and 1-5 people, are both 39psi. After gas prices really started to take off in our area I decided to raise the tire pressure from 32psi to 39psi and see if that would improve my gas mileage. It did.
I went from a little more than 24mpg to almost 26mpg. Increasing the tire pressure 7psi gave me basically 1.5mpg.
There is a tradeoff for this gain. The ride is much stiffer with the increased tire pressure. It can border on uncomfortable at low speeds and I notice it the most as I roll into the garage. But a lot of people like the “stiff suspension” feel and I think it is definitely worth it for the 30 extra miles per tank of gas.
More about mpg in the SAAB 9-5:
The EPA has my 2000 2.3t with premium fuel at 18mpg city and 25mpg highway. My SID stays between 25-26mpg combined. I drive about 40 miles every work day on the highway and 20 miles every workday in the city.
Of course, your mileage may vary.