91 300E Trans Gear question

[JRR4948]

I have a 1991 300E 3.0L. At 65 MPH the engine is turning approximately 3,000 RPMs. I am getting 25 MPH on highway mileage and 21 combined. Can I have a different top gear installed to drop the rpms and save fuel. This car has amazing torque at high speeds, I am assuming this is so due to the high RPMs. I am not looking for performance. My other cars have similar transmissions w/o overdrives yet still turn at 2,000 RPMs at these speeds. Also how much should this cost to do? Thanks JRR