There are a lot of things that effect gas mileage.
RPM isn't the ultimate evil. Using the most efficient part of your power band to accelerate to speed, then using the least amount of engine power to
maintain that speed will give you better gas mileage.
Idling is your worst enemy. You are getting infinite negative mileage, since you achieve no distance for the fuel you consume.
Cool temperatures are good, hot and cold are bad. Too cold thickens the oil (more resistance), spend more time warming the car up (idling), and can
lead to bad road conditions (snow and ice). Too hot, the biggest hit is a/c, which can lower mileage 20-30%.
Driving habit makes the most difference. This combines minimizing idling, coasting as much as possible, and using your brake and accelerator pedal as
least as you can. Conservation of momentum is your ally. When you use your brakes, that fuel you burned to build up kinetic energy gets turned into
heat by your brake pads and rotors. So, you see a light will turn red before you get to it, or a stop sign coming up, let off the gas early and coast
as long as you can. For a red light, any left over speed if it turns green before you reach a stop helps a lot. Going speed limit can help a lot
too, most cars are the most efficient in the 35-55 mph constant speed range.
Again, idling destroys mileage. This is why Hybrids have 'auto-stop'. The engines turn themselves off at stop signs and red lights (some turn off
under 5 mph under braking), as to not waste fuel while sitting still. If you allow the car to start back up, you can watch the mileage drop.
Fuel is a factor as well. The octane rating shows its ability to resist detonation. So, 87 octane is more combustable than 93, but it also can lead
to pre-detonation which is bad for valves, rods, pistons, etc. Early ignition can come from the cylinder being too hot or high compression, among
other things, and this is why high performance cars usually take a higher octane, to protect the tuned engine that usually runs a bit hotter with a
higher compression. As others have stated, Ethanol has less energy per unit, so for the same amount of fuel injected into the engine, less power is
produced, which can lead to a lower mpg stat.
Tire pressure also plays a big role. 20 psi will take a several mpg hit. Factory recommendation is better. Though not advocating it, as the tire
pressure increases, the rolling resistence decreases, so the higher you feel comfortable with the better. At a few sites where people try to maximize
their mpg, some pump up to max cold pressure on the sidewall, some even go to the 50 and 60+ psi in each tire. They notice more even, and less wear
on their tires. The difference between 40 and 60 psi is minimal compared to the difference between 20 and 40 psi. So keep your tires to at least the
factory recommendation, if not close to the tire's max cold pressure.
Extra weight. If you have a trunk and cabin full of stuff, it takes extra energy to move those items. If you do something such as add bigger,
heavier rims and tires, then it is magnified, since, rotational weight takes more energy to accelerate than dead weight (just as replacing a 24 lb
flywheel with a 12 lb flywheel will help more than you going on a diet and losing 12 lbs).
Cars can increase in efficiency in the first 10-20,000 miles. If well maintained, the amount of power loss is minimal, especially if you don't
'drive it like you stole it', excessive rpms does create excessive wear in most non-rotary vehicles, rotaries do seem to perform better when driven
consistently more 'spirited' (3000 RPM+ to below redline), though that isn't good for mpg!
If you have been topping the car off when you fill, you will already have an idea how many gallons you should be putting in compared to what the gage
states. Occasionally there is a pump somewhere that is messed up. But, the state isn't helping them screw customers, for, they make quite a bit of
money from fines if the company intentionally messes up a pump to their advantage. Every pump is to be inspected every year, even tested to verify
that the premium is premium and the regular is regular. Most mid-grades use a mixer to combine the two grades so they don't have an extra
underground tank just for that.
So, in the big picture, how do you drive? Do you conserve momentum? Do you keep the lowest weight oil in your car that is recommended? Do you have
excess weight in the car? Do you keep the car in good maintenance, including tire pressure? Do you live in hilly or traffic congested areas (has the
route changed as such)? What is the weather like? Do you use a/c a lot?
I drive a Civic Hybrid. I started out in the low 40s and high 30s on a bad day. Now I consistently get 50+ and mid to high 40s on a bad day. I have
highway trips in the 40s going excessively fast, 50s-60s going the proper limit, and in the 80 mpg range going 55-60 mph on a long trip, over 1000
miles on a 13 gallon tank.
I have also gotten 50 mpg out of a 4-door Accord EX on a long trip.
What I am trying to say, it is more than just what RPMs you go up to, but a whole array of variables that influence your mileage ... but the biggest
factors are the driver, the maintenance/health of vehicle, the fuel, and the accessories you run.
Check out CleanMPG.com
and read through their threads and articles for a much more in depth take on these things.
Some of it is overboard, but the general idea is conservation of fuel to save you money no matter what you drive, be it a F150 or a hybrid. Actually
I think their motto is 'learn to raise fuel efficiency and lower emissions in whatever you drive'.
You may be able to bring that Monte Carlo up in the mid 20s to low 30s city and above 30s mpg highway with just a few minor tweaks and adjustments to
your automobile ways.