91 940 turbo sedan.....what is better for gas mileige...driving with the little airrow (overdrive switch?) in the dash on or off?....and where would my oxygen sensor be located? If anyone could help me that would be great.. thanks!!!

 

Make sure the arrow is off then you have overdrive.If the arrow is on you don't have overdrive.

The O2 sensor is located in the exhaust a little bit back from the engine.