I did that for 80 MPH because I know the RX8 engine runs
at 4000 RPM in sixth gear at 80 MPH. I calculated the
fuel burn given the .67 BSFC at about 30 pounds of fuel
per hour. That is about 5 gallons an hour or about 16
MPG. Not far off what an RX8 will really do at a steady 80 MPH.
My measurements using the real time OBD II data indicates
the RX8 will get roughly 20 MPG at a steady 80 MPH.
You can calculate the pounds of fuel burned by dividing
the OBD II mass air flow data by 14.7. 14.7 is represented
as one in the OBD II charts. I have included some for
reference.

If you changed the RX8 rear axle ratio for 3000 RPM at 80 MPH
the BSFC would drop to 0.57. Fuel burn would drop to
15.6 pounds per hour or 4.3 gallons per hour and MPG
would jump to 18.6. That is about a 16% improvement
in steady speed MPG.
Paul Lamar

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 Paul,
My "91" RX-7 gets 24-26 mpg, @ 3000 rpm, which for my car is 80 MPH.
This is actual driving in the high desert of central OR. 3500' Elv. I go
into the valley all the time, going over the Cascades to get there. I
usually cruse @ 80- 90, sometimes pushing to 100 to get around trucks. I
have never seen mpg below 21.5 mpg, that is mostly when I do just city
driving.
I don't know what the rear axle ratio is. This car has 140,000 miles on it,
and these mpg's are all with the top down!  (;.)
I keep telling myself, some day, I'll drive at a steady 60-65 mph, just to
see what mpg I get. And then I start driving again.
Tom Phy

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I had no idea the 3rd gen RX7 was/is geared that high.
Of course it is a turbo so it generates more torque at 3000 RPM.

I wonder why Mazda geared the RX8 at 4000 RPM at 80 MPH?

As I said the BSFC chart I was working with is 50 years old.

A friend just drove his RX8 to Las Vegas and back to LA
claiming to hit 103 MPH and averaged 21.4 MPG.

Thanks for the new data.