Q: Our 2004 F-150 with a 4.6-liter V-8 and 3.73:1 gears lost mpg when I changed the tire size from 235/70R17 to 265/70R17 (Fuzion LT tire). I was hoping a taller tire would reduce rpm and improve fuel economy on the highway. I was dead wrong. It went from 20 mpg down to 15. I purchased a Predator by Diablosport to adjust for tire size and correct speedometer and shift points, using the 87-octane setting. Would a 3.55:1 gear help?
I can't seem to find a Fuzion LT model tire, but tread pattern, rubber compound, weight, height, and width play a big part in rolling resistance, and therefore fuel economy. I'm not sure on the tread, but you did significantly increase tire width. Technically, increasing tire diameter (within limits) while maintaining width and tread should increase highway fuel economy (how much cannot be estimated without testing). At the same time, the larger tires will decrease city mpg. So upsizing tires for fuel economy shouldn't be attempted unless doing strictly long-range highway miles.
You could be getting too much city driving included in your highway mpg calculations, or the tires you've chosen are cancelling out any gains by the increase in height. Or it's possible you're driving too fast -- your truck's PCM is now receiving incorrect vehicle speed data due to the tire upsize, and so are you. If you're driving at the same speeds as before, you're actually driving faster and negating any highway mpg gain.
If you're planning on sticking with these tires, it's a good idea to reprogram the PCM to adjust for tire size. Changing gears is not going to help. And even if it did, how long would it take to cover the price of the gear installation at the pump? In the design of car and truck powertrains, engineers do extensive research before deciding on transmission gearing, axle gearing, and tires for a particular model. It's all in an attempt to reach a balance between fuel economy, emissions, and driveability.
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