Hi,
I recently purchased a 2001 f150 with The 5.4 Triton. The truck has 210000 miles on it, and is getting around 8 mpg. I don't know a whole lot about the history of the trucm, but the guy said he recently replaced the intake manifold, spark plugs, and ignition coils. There are no check engine codes. The only codes that are tripped are ABS codes related to the control module. The truck has big 35s on it, but I can't imagine it would drop the gas mileage that much. Is there anything that I can do in order to improve the mileage. I'm pretty sure that there has to be something wrong in order to get that bad of gas mileage.

 

35's and lift kits will kill gas mileage, and I'm betting that the speedometer/odometer hasn't been corrected for the oversize tires, so you are traveling farther than is indicated.

 

That and check your spark plugs and coils. I recommend MC.

 

You can be assured of one thing, buying an unknown with larger tires and noncorrected speedo will not give you near correct average mileage under the standard way of calculating it.
Besides it takes more throttle to push higher rolling resistance and reduced final gear ratio due to the larger tires. All a stack up against fuel mileage.
Good luck.

 

I have a 4.6l 4x4 supercab, longbed. Got 8mpg all around and had a check engine when I got it. Turned out to be a DPFE and I replaced some other emissions hardware that was old. Did the Gotts Mod with cleanable air filter. Put tire pressure at 37. At this point it was running about 10 around town and 15 or so highway. I think I did plugs then and got 10-11 around town and maybe 16/17 on the highway but I have not been exacting enough with refill/mileage comparison. I recently had a pinion leak and fixed that up, new oil. The truck rides and accelerates differently, coasts differently. Is the rear end oil a big deal? I dunno how long that leak was going on but nothing in the driveway until it started gushing from what I can see. Just seems like a big change in the truck after the seal repair and oil change in the rear end. Anybody have this impact gas mileage? New oil in the back end that is? What about the front gear box, does that help?

 

35s with stock gearing will kill the mileage. Your speedometer will also be way out of calibration. Run your Waze app and see how fast the truck is actually going vs. the reported speed on the speedometer. You can also watch the side of the freeway, or state highway, for the milemarkers. Observe the odometer as you pass a marker, then see what the odometer reading is after ten miles. My guess is you will see about 8 miles roll by on the odometer, and the speedo will report 60 ish when Waze says you are doing 70.

If the truck has 35s and stock gearing, it should be a complete dog on the highway, and you might even have to shift out of overdrive. If I am right, your cure is go back to factory tires or get the truck re-geared to lower gear ratio (higher numeric value). I'm thinking you have 3.55 gears, very common on the F150, and your tire package should have you on a 4.10, or more. Read up on differential gears. My '97 F150, 4.6L Triton, gets the 3.55 limited slip differential. My tire package is 30 inches diameter.

 

Just to toss this in there, I am maxed out @ 90mph via gps, and my speedo is maxed over 110 mph. I have 35s and a stock, listed ratio of 3.55s and a limited slip.... I will be getting a tuner soon to fix things.....

I did the jack the tires off the ground thing, and think I have 4.56! gears, as my driveshaft turned 4.5 times per the tire turning once