JUST THINKING OUT LOUD about lessons learned by trial & trial....errr...trial & error:
Well.....a while back I replace the 3.44 diff gears with 2.93 Mustang gears in order to get better mpg's....
now.....here's the thing...the vacuum gauge shows that the engine lost an average of 6 lbs. of vacuum in everyday local driving at 50 mph or less when the transmission is in 4th gear Overdrive....
which makes for about 12 mpg (down from 16 mgh w/3.44 gears

)
BUT....the good new is....the 700 R4 transmission has 4 forward gears.....
AND 2 overdrive "gears"(T.C. locked!).....3rd gear & 4th gear lock the torque converter when the 12 volt switch is engaged to the transmission...
which means if I leave the transmission shifter in 3rd gear that I can drive around all day in 1st/2nd/3rd/3rd O.D. with the T.C. overdrive switch turned on....
annnnnnnnnnnnnnd at 60 mph on local highways, the engine only turns 2500 rpm---which is the same rpm that the engine turns in 4th gear with the T.C lockout turned off....
which allows the engine to pull a strong 18 lbs. of vacuum in local city/expressway/stop/n/go driving....
which makes a tank of gas go a lot further during local driving....
annnnnnnnnnnd the other good news is.....on road trips...it gets 20+ mpg when I used the 4th gear overdrive (T.C locked)....
So.....my undergearing mistake turned out O.K....and with the external transmission fluid cooler...NO OVERHEATING
