If you overinflate your tires by about 30-35%, you can significantly improve mileage as you will reduce rolling resistance. Just don’t do it in the winter.
Just don't be surprised when your tires go bald in the center of the tread.
Debunking a Mileage Myth: Can You Really “Pump Up” Your Fuel Economy?
http://www.popularmechanics.com/cars/news/4199963
The door placard on the Honda recommends 32 psi in all four tires. The sidewall says the maximum pressure for the tires is 50 psi. So for the trip to Arizona, we inflated the tires to 45 psi. We figured 5 psi under that maximum inflation pressure would reduce drag enough to boost our mileage by some measurable amount. Before reaching Phoenix’s West Valley, we were able to travel 394.1 miles, then filled up in Glendale with 9.34 gallons and recorded 42.19 mpg. That’s the best mileage we’ve recorded so far in this car.
Two days later, we dropped the pressure back down to the recommended 32 psi and pointed the Fit’s nose home toward L.A. This time, over the exact same route, at the same speeds, the Fit went 411.3 miles. At the Chevron station in Santa Monica, our Fit drank 9.76 gallons, translating into 42.14 mpg—nearly the exact same mileage.