I currently have a 87 f150 ext cab 4x4 with a 351m,c6 trans with 4.11 gears. I currently have 31" tires on it and am considering 33" tires my question is if I dont change my driving style will I get better or worse fuel economy.(my 31's are shot and I was told 33's will give me better fuel economy with my current gearing)

 

With the 4.11s and the C6, you might inprove it a small amout, but as a general rule - bigger tires = less mpg.

 

Since you have the c6 I assume you don't have OD which in that case I bet your mileage will increase dramatically. Especially with those 4:11 gears.

 

It all depends in the motor can push the tires with ease, and with 4:11 gears, you should get better mileage with 33 because you'll be turning less RPM's than with a 31 if your motor can handle it, and I don't see why it would have a problem being a 351