You ask a question that, well, the answer you want depends on MANY variables.
The best fuel economy depends on several things (and int in any particular order):
1) Condition of engine and filters (
plugs, etc)
2) Type and condition of tires (low rolling resistance tires, etc)
3) Tire INFLATION pressure. When doing long highway trips I tend to run a few pounds to the harder side (but not much).
4) Terrain. Obviously FLAT terrain will provide overall better fuel economy than hilly or mountainous terrain. Hilly terrain does not 'even' out economy because there's always a heat and friction penalty to pay when going uphill (think of raising a weight to a height and think 'work'). There's no such thing as a free lunch.
5) Weight of vehicle (excess weight penalizes you when accelerating and braking).
6) Driver experience and habits
7) Use of air conditioning and other equipment, depending on weather. I don't recommend driving with windows open, however if weather is pleasant I will run without AC on and just have air exchange in the cabin.
8) WEATHER. Head winds INCREASE fuel consumption, and tailwinds decrease fuel consumption as the air resistance decreases with a tail wind. Cross winds have little effect EXCEPT to the extent that a cross wind (other than a 90 degree crosswind relative to your direction of travel) has either a head wind component or a tail wind component. Think of driving into a 40 mile headwind and you are doing 70 mph (ground speed). The air your car has to push out of the way is coming at you, now, as if you were doing 110 mph. Obviously you are not expending the fuel as if you are doing 110 BUT the extra resistance from pushing that faster air out of the way comes with a penalty.
Temperature and relative humidity also affect fuel economy. A regular gasoline-powered car sees its gas mileage drop by 12 percent when the temperature is 20 degrees. COLDER temperatures mean that the air is DENSER. Not only is COLDER air harder to push out of the way, it affects how the car's ECM meters out fuel, etc to the cylinders. Colder air is denser, which means more oxygen. Without the ECM adjusting the injection process to provide more fuel, the engine will run lean, so it adds a bit more fuel by allowing the injectors to stay open longer to compensate for the additional oxygen.
In conditions where relative humidity is HIGH, you will may, but not noticeably tend to get better fuel economy - except where humidity and HEAT force you to use A/C. Depending on the engine type you could get a very slight boost in efficiency due to the increased humidity in the engine's air intake. Also, the increased humidity of the air while it is raining will decrease the density of air. This will result in a slightly lower dynamic pressure and thus slightly lower drag.
Altitude also affects fuel economy (see terrain). The ECM will compensate fuel injection and timing to make up for variances in altitude.
I lived in SE Arizona for 25 years, and bought the Genny in 2011 (3.8L). We moved us and it to OH a few years ago. IN Arizona we had to deal with very HOT, very DRY conditions, as well as living at 5,000 altitude. The best I was able to get out of the 3.8, considering a trip to Tucson was 80 miles (downhill from 5,000 to 2300') one way was somewhere around 28-30 mpg. This was the average for the entire trip 160 miles, down and back up).
9) Fuel blend: Winter or Summer blend? Winter blends have more additives, etc to reduce emissions (and economy).
10) E10 vs '100 percent gasoline'. Virtually all fuel comes blended now. There are places where you can still get "100% gas, but suffice it to say that an E10 rating at the pump (90% gas, 10% ethanol) can vary by as much as 5%, and the less alcohol, the better economy you may notice. Ethanol provides only about 75-80% the BTU content of a gallon of fuel. Without doing specific math and numbers, think of it as this: In a give amount of gasoline, say 10 gallons, you have about 110,000 BTU per gallon of energy available (making it simple). If you pull out ONE gallon and replace it with ONE gallon of ethanol, you are replacing that gallon with about 80,000 BTU (I'm making rough estimates for demonstration sake). So you do take a penalty in economy and performance but NOT a big one. Think of it as replacing 1/10 of your fuel volume with a gallon of alcohol that is 80% efficient. So I think the overall penalty is about 3%. If you add to that the fact that winter weather and winter blended fuel is used, you CAN lose as much as 10% economy, but overall, just e10 fuel can cost you 3-4 percent or so. Maybe as much as 5% if the blend is well over 10% (15%). So you can pay more for '100% gas" if you can find it. Unfortunately, about the only places you can now are near marinas (for boats). Aircraft still use unethanol'd and leaded fuel and AVGas is no deal. I see that some stations like Sunoco offer 100 octane racing fuel (I'm thinking that's all gasoline) but they want like $7 a GALLON for it. NO deal. I think you can go online and look for stations around you where you can get 100% gasoline if that's a big factor to you.
Hope this helped answer your questions. Some variables you can control, some you can't. In Arizona, where the speed limit was 75, I ran anywhere between 65 and 80 depending on weather, terrain, and wind. Here in Ohio I drive 25 miles to work, some backroads, some freeway and some city streets, and average about 25 mpg.
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2014 R-Spec here. Going on a long road trip next week (1K miles round trip). Car has 49k miles on it now with new shoes. About to go get an oil change as well.
Curious - what have you all found is the ideal speed for crusing on road trips for optimimum MPG? Also, what tire pressure for MPG / comfort balance?
Tonight I did a 75 mile road trip. Filled up with regular (I typically use Premium 91) just before leaving. Had the cruise set on 69 (speed limit is 65) the entire time and averaged 25.1 MPG. Not too bad, but I think there's room for improvment.
Thanks in advance for the feedback!
