Yes increasing the tyre pressure to 34 psi will negligiably decrease the contact area, elasticity and tyre compression. But it will decrease the work load for engine and road friction increasing your FUEL ECONOMY.
Increasing the spark plug gap to 1.5 mm will increase the magnitude of spark. The air fuel mixture will be burnt more effectively giving your engine more power for the given amount of air fuel mixture. Thus again increasing your FUEL ECONOMY. However if any of the plug is weak, there are chances that the plug may misfire and your engine may start missing. The chances for misfiring of a plug increases as the plug gap increases beyond 1.5mm....