I changed out the front wheels, 40 psi cold.
185/65R15 vs 165/80R15 (165R15)
The 165/80R15 tires give 70mph on the GPS at 65 mph on the speedometer or 7.7% increase. The car seems to slow up hill, but considering a head wind and 450 lb weight of passengers, not too bad. You defiantly want to drive slower with the skinny tires, since the car doesn't handle as well as the 185/65R15 tires. It seems to coast really far, though. Almost down to 1/4 tank.