Okay I need direction of this. I took my truck (01 F-150 4X4, 5.4L) in for a tune up (replaced the plugs, 1 COP, and cleaned the injectors) about two months ago. I also replaced all 4 tires the same weekend (318x75x17). Since then I have started to notice that I don't get the same fuel economy that I use to. Now before everyone jumps in, I know it’s a 4X4 and doesn't get great MPGs anyway, but it seems to be worse them before. I would appreciate any suggestions, thanks.

 

35's with stock gears are what is killing you mpg's because the trans is not shifting at the right points.

 

How bad are we talking? Did you increase your tire size when you got new ones? Have you adjusted your speedometer/odometer for the larger tires?

 

I can't put a definite number on it but I would guess at around 1/2 to a mile a gallon. I previously has 305X65X17s, the new tire are bigger but not much. As for the gears; is that something I can fix with a programmer or do I need to change the gears in the differential and what gears sizes would you recommend?

Thank you both for responding.

 

A little pricey, but you can compensate somewhat with a custom tune (SCT or Edge programmer) - which will also account for the tire size difference from stock (235 or 255), and change your shift points. You may not see that much MPG improvement (maybe 1 or 1.5 MPG, dependent on settings), however, your tranny will thank you in the long run. I have an older Edge Evolution, where I used a level 2 program (assumes towing, which I am not), which did address a lean mixture (due to K&N cold air intake), and agerage MPG went from 13.8 to 15.3, under the same no-load driving conditions. The SCT allows for more variables, so it might get better results!