This was posted a bit back over on Renntech:
--------------------------------------------
You can calculate the approx ground speed at fuel cut off for all gears as follows:
5 sp. forward gears (std 17" tyres)
("current engine rpm"/1000)*5*("transmission gear#")= "x" mpg
i.e. for 3rd gear at redline (7200/1000)*5*(3)=108 mpg
I've noticed that interesting relationship before while driving; the gears are almost exactly evenly separated so you can approx groundspeed knowing only rpm and gear. This is for the 5sp only on the Boxster std.
---------------------------------------------
I agree for street driving why would you need a S ;-)
*** Now for all the owners of an S to chime in
****