Lowering the rpm's will decrease power. This rpm for this speed sounds normal, so why are you trying to lower it. Fuel economy? The engine may work harder at a lower rpm and actually use more fuel. You can lower the rpms by changing the rear axle ratio. This can be done in 2 ways. Remove and replace the differential with a different ratio unit, or change the size of the rear tire/wheel assembly. Going to a larger tire will decrease the rpms, but not by much and if will cause the speedometer to be off, and possibly effect the computer settings. One other way is to install an overdrive unit, which would be very costly.