This data suggest that merely installing an overdrive
unit on an RX8 will improve the MPG to 35 on the highway.
The RX8 engine is turning 3000 RPM at 80 MPH.
A Corvette turns less than 2000 RPM at 80 MPH and gets 25 MPG.

Both cars need less than 40 HP to go 80 MPH.

http://www.superchevy.com/technical/engines_drivetrain/transmission/0411sc_od_copy/


"It's been 2 decades since Detroit began offering overdrive transmissions as original equipment in virtually all vehicles rolling off their assembly lines. And while the OEM's intent was to improve fuel economy <http://www.superchevy.com/technical/engines_drivetrain/transmission/0411sc_od_copy/#> and decrease engine wear, it wasn't long before enthusiasts realized the performance gains that were possible by adding another gear with a ratio of less than one-to-one. With overdrive in high gear hot rods could benefit by using a lower (higher numerically) rearend gear, which translated to greater acceleration without the inconvenience of higher rpm while cruising at highway speeds. This was akin to having your cake and eating it to, as high-output engines were allowed to run in their power range while blasting from stop light to stop light, only to be given a reprieve when it came time to hit the open road. "

Paul Lamar