Need Better MPG?

Question: My 2004 F-150 with the 4.6-liter engine and 3.73:1 gears lost mpg when I changed tire size from 235/70R17 to 265/70R17 Fusion LT tires, hoping a taller tire would reduce rpm and give me better highway mpg. I was dead wrong. It went from 20 mpg down to 15. I bought a Predator computer programmer by Diablosport to adjust for tire size and correct the speedometer and shift points, and I used the 87-octane setting. I'm open to your suggestions. Would a 3.55:1 gear help?</i>

Answer: In theory, increasing tire diameter, with all else being equal, should increase highway fuel economy. The change in final-drive ratio will reduce engine rpm at a given vehicle speed and load. At the same time, the change in ratio may hamper city mpg by increasing engine load at lower speeds. That extra oomph from smaller-diameter tires increases efficiency in stop-and-go driving. Gain a little here, lose a little there. The first number you see in a tire size (235-265) is the tire's width in millimeters, and the second number (70) is the aspect ratio--meaning the sidewall height is 70 percent of the tire's width (235-265 mm). Your upsize from 235/70R17 to 265/70R17 increased tire diameter about 1.6 inches and tire width about 1.2 inches. Increase in width adds to the tires' area of road contact, which increases friction and decreases fuel economy. Installation of tires with a coarser tread pattern also ups the ante for friction and fuel use. There are a lot of variables to consider: There may be inaccuracies in your highway mpg calculations and/or the additional width/mass/new tread pattern had an effect on any highway mpg gains from the taller tires. Installation of an aftermarket programmer to update powertrain control module software with the new tire size was the right way to go, but I wouldn't suggest spending money on new rear axle gears unless you do strictly highway driving. I'm talking nonstop 18

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If you don't already have one, I'd suggest a tonneau cover. Granted, the mpg gains aren't nearly what the companies claim (5-10 mpg or other such ridiculousness) but they will provide an incremental improvement. But you have to crunch the numbers on the cost of the tonneau vs. how much you drive and your potential fuel-economy gains to determine whether or not it's worth the cost.  

i have a 2000 f-150 i added a cold air intake and a throttle body spacer and mine went up from 19 to 21