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Being a new owner of a 2007 FJ 4x4 ( w/ 157,000 miles) I've been curious about what I'll be seeing for MPG.
The rig is lifted with 285/70/R17 tires and an ARB front bumper w/winch, so far from stock.
My first couple tanks hovered between 16-17 MPG, not bad for winter driving in N.E Mn.
Yesterday I had to run to Duluth, 260 miles roundtrip, averaging about 60 MPH. Temp's were in the 20's yet I got 19.1 MPG,
about what we see with our 4Runner and Taco on a good day.
After all the FJ horror stories of poor MPG I was quite pleased.

Automotive parking light Wheel Automotive side marker light Tire Land vehicle
 

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Keeping the speed down to 60mph, and avoiding stops, does amazing things for mpg. My supercharged FJ, which normally gets 12~14mpg, got 21mpg one week driving long distances on 2 lane roads where 45mph was the posted top speed (due to wildlife).

Wind resistance goes up by the square of the speed, and it begins really adding up above 45mph.
 

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2007 Silver FJ ~130k
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The lack of ethanol helps. I got my '07 in March of '06 about six months before ethanol was put into the gas in DFW. I was getting approx. 17 around town pre-ethanol and post ethanol it dropped to about 15 and I use premium.
 

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I've recently swapped to 90-octane, ethanol-free and I'm seeing an extra 1-2mpg over the regular 93 octane. My commute doesn't change, nor does my mpg/range normally, so it's definitely the gas.
 

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Aerodynamic drag ('wind resistance') increases as the cube of the speed, not the square, so any increase (or reduction) in speed will have a significant affect on MPG compared to some 'baseline' speed.
 

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2012 FJ Cruiser Trail Team
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The same theory as density altitude with aircraft lift applies. In cold weather the air molecules are condensed so the engine is receiving more air molecules. Warm/hot air the molecular are expanded. Cold air in an air intake is more efficient.
 

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All of those things are true, but dropping down from 75mph to 60 for a tank of gas makes an immediate and huge improvement in mpg.

10% ethanol is only 3% less energy than pure gas, so it is a modest impact to mpg.
Fuel octane can make a difference, but its usually quite subtle too.
Cold air vs. warm air does make more power, but most people don't see a 3mpg increase in winter, like OP saw when he drove slowly for a full tankfull.
 

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I got my best mileage in Yellowstone where the air is about 25% thinner (7000-9000 ft elevation) and the speed limit is 45 mph. Despite having all my stuff in the back and carrying two bikes, I got 22 mpg.

If you have rolling hills on your drive, you can use them to get even better gas mileage by accelerating downhill and letting off the gas going uphill. If the speed limit is 55, I go 60. If I see a hill coming up that dips down first, I go up to 65 downhill and then fall down to 55 climbing up the top depending on the size of the hill. For bigger hills, I might go up to 70 and fall down to 50. You can get 10-20% more mpg depending on the hills. I get really bored during long drives so if someone isn't following right behind me I might do this.
 

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Something must be wrong with my FJ! I get 11mpg average as of today, and I only run premium!. 6 speed. Granted I do a lot of idling, but all the major maintenance stuff- air filter, all fluids, spark plugs, etc are all up to date. Maybe I just drive too fast on the highway :)
 

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Something must be wrong with my FJ! I get 11mpg average as of today, and I only run premium!. 6 speed.
After checking the obvious things like a dragging brake (hot rim after driving), very low tire pressures, etc, try just relaxing in the slow lane at 55mph, maybe with the cruise control, and you might be surprised how good the mileage can be.
 

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In the DFW area and all around texas, if I go on the highway at 75MPH, I'll get 14-15MPG even drafting box trucks. Going up into New Mexico or Colorado, keeping the speed at 65, even with boxes on top of my Cruiser, I've gotten up to 16ish MPG. I have 305/70r17 tires which are ~34x12" tires running 32PSI for good contact patch, and I'm part time 4wd (not full time 4WD like my wife's '07 stick shift FJ). Cruising around town on state highways and surface streets, I'll usually get just under 15mpg. I could probably get another 1-2MPG if I put the tire pressure up, but I'd rather have the grip if I need it. I've just come to accept that my FJ gets awful mileage and has a teeny gas tank hahaha
 
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