MPG issues/ Tranny question...
 

1989 300te...sluggish...I had all systems checked, mechanic said all was ok...15 mpg. Says the O2 sensor is ok, mixture is fine. Any ideas? I would love better mpg and peppyness.
Transmission: Can I shift to 3 while driving in order to get more ummph? As long as the rpm's don't exceed their limit. Let's say I want to pass, merge, go uphill...

Doing a stall speed test will tell the health of the transmission and most importantly the torque converter. The stall speed of you car should be around 2200RPMs, anything higher = the transmission or torque converter is slipping.
The most accurate method for determining actual stall speed on a car is to launch the vehicle at full throttle, flash stall and note the rpm at which the car actually takes off (requires a partner watching the tach). the owners manual will tell the speed of each
selected gear.