I have a 1989 F150 XL 4.9L 300 Inline 6 with a 4spd man tran. The problem I have is poor gas mileage about 10 MPG. I've had it checked by a mechanic, replaced plugs,wires, dist cap rotor, ignition coil, and have also added fuel additives and no change. My truck had 2 cat converters but I removed one and I was told too remove the other and just run straight exaust allowing my engine too breathe inturn improving mpg. Question 1. The same guy told me too take off the the other converter , knock out the guts and reweld it to the exaust pipe. WILL THIS AFFECT MY O2 SENSOR? Question 2. What else can I do to fix this other than the above?

 

It will not effect the O2 sensor but you will lose what little off the line torque you have. I would install a new 2.5" hi-flow cat convertor, a new fuel filter and ensure the plug gap is correct. I would also swap gear oil in the differential and perform a trans fluid & filter service, use 10W30 oil and keep a fresh air filter installed.

Fuel additives will actually errode the plug electrode prematurely if used on a regular basis. It is better to run a quality gas such as Conoco, Shell, Chevron or QT.

You can play with the plug gap and tire pressure to see what gains the best mileage. If you are running Bosch plugs I would recommend replacing them for Motorcraft or Autolite copper plugs as they are a better plug than the Bosch.