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


good data, paul. is there any real reason all cars arent manufactured with gearing that would allow highway speeds (today's version meaning approx 85-90 mph, eh?) with rpm's significantly lower? I often cruise around side streets with my rpm's at 1200 doing about 30mph and it seems fine, so why cant we all be running 1200 rpm on the interstate? just curious about this, seems like everyone wants to claim we'll run out of oil etc etc and such so if these are real problems then wouldn't low rpm interstate geared vehicles be something to consider?

jess loop

--

Most cars are geared like that now. My 2004 Honda Odyssey is doing about 80 MPH at about  2000 RPM.
So too a Corvette.

The rotary engine is a high HP, high RPM, low torque engine. It is only 2.6 L. The Honda is 3.5 L.
Mazda feared criticism for bad acceleration at cruise speeds so they kept the high rear axle ratio
at the expense of lower MPG at freeway speeds.

What  the RX8 really needs is an electro hydraulic mechanical gear box that will shift automatically.
Now found in high buck Euro cars like the Ferrari and Formula 1.

That would put the price out of sight so the auto down shifting over drive box would be cheaper.

Paul Lamar

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