Yes, the OD saves you gas, but if your tranny is "hunting" back and forth between 4th and 5th gears (like in the situations others cited) then turning off overdrive saves some wear and tear.
Also, if you are going to be wanting to do some sudden, high speed passing, then turning it off before you get to that point eliminates some lag in the downshift when you really hit the gas. At some speeds, your tranny will want to shift back to 3 to pass and to come down from 5 to 3 takes more time than from 4 to 3 -- or so it seems to me. I use it that way sometimes.
The CAT's have been strung out like that for efficiency. It allows lower emissions with less restriction by stringing the catalyst out over a longer distance. The earlier smaller cats also pre-heat the exhaust gas (the catalytic reaction releases heat) making the big cat even more efficient.
It's not worth it to mess with it and it's always possible that emission standards will get tighter and then you'll really be up the creek.
Last edited by n3elz; 03-03-2005 at 09:57 PM.