I have an 04 FX4 with a 6" Fabtech, 18" wheels with 325/65/18 terra Grapplers..I get about 8MPG right now...This is a new truck to me and I expected less than average gas mileage but is this common and if I was to go with a "closer to stock" tire size would it help my MPG at all?

 

Has your speedometer been corrected for the size of the tires? If not, that is probably why it shows as 8mpg. Also, if it has lower gears and not been corrected for that, it would also be off for that. As to stock tire sizes, yes they would give better gas mileage as they would have less rolling resistance. However with a lift, they would certainly look funny.

 

I bought the truck from a guy who didn't have much knowledge of the upgrades..I basically saw a great deal and I jumped on it, As for the programming..That is what i suspected initially because I didn't think the mileage could be "that" bad. I will say that the truck shifts at very low RPM's and I can do about 65 at 1800RPM..Should I invest ina programmer or take it somewhere and have the settings checked? Appreciate the help, this is my first "real" truck..

 

The dealer will not have anything to be able to program for that size tire. They have a limited selection they can program for. Your best bet is a programmer. They do have some that are not full tuners, that are a bit cheaper.

 

I am looking as we speak, I have had other trucks and most recently two Expeditions, but I can't beleive that the gas Mileage is taht bad, and I drive pretty conservative too.

 

Another question, what tire pressure are you running those tires? Nitto has 2 load range D tires in that size. One with a max of 50, and one with a max of 65. If the tire has a max of 50, you should probably be running them at about 36. If they have a max of 65, they should be run at about 45.

As to the size difference, if your truck originally came with a 275/65R18. If uncorrected, the speedometer would only show about 65 miles for every 71 you have actually traveled. Also, with an uncorrected computer, the trannie would not have the most efficient shifts.

 

get a programmer, you can fix the speedo, get more power, and improve your shifts.

 

Have you checked the mileage manually or is it showing up on the computer from the truck?

BTW, I could get almost 500 miles out of one tank on my Expedition.

 

Those bigger tires will affect the MPG significantly. If the wheels are aftermarket and bigger they will also. All of the additional weight makes a difference.

 

You can't rely on the computer or for that matter the odometer with larger tires. Go fill up at the gas station and use a GPS to track mileage for the tank. Fill up and do the math manually.