I believe your acceleration experiment may be poorly designed (though I may be wrong since Jeeps may be more like trucks).
It is my understanding that car engines operate most efficiently usually somewhere between 3000 and 4000 rpms (for trucks, the sweet spot is usually between 1500 and 2500 rpms). Your most efficient speed, with little fuel and time trade off, should be somewhere between 50 and 80 mph. In your case, it looks like somewhere around 50 or 55 (loose guess based on your graphic). See what your RPMs are at that speed. Try keeping your RPMs near there when you accelerate (within reason consindering the conditions).
Also, try that just in first gear and compare it to idling. Measure your volume of fuel used and distance traveled. Measure over the same distance, starting from maximum idle speed in both cases. Do it again starting from a full stop.
Take a look at incremental fuel consumption over incremental speed.
0.062733333 0 to 45mph (this covers multiple gears, so doesn’t compare to other numbers)
0.0966 45 to 55 mph
0.121 55 to 65
0.1653 65 to 75
It looks like you still get a very slight increase in fuel consumption for an increase in speed then you’re down near 45 mph.
It would be cool to see narrower speed increments for the entire speed range of the gear.
I think it would also be good to note that this analysis ignores any time and traffic considerations. It is only relevant in conditions where you are the only vehicle on the road (your might be able so save fuel yourself, but at a cost of more fuel consumption for others).
If you really want to save fuel and still drive, stay in first gear at about 3400 RPMs and avoid stopping.
The AMC I6 "sweet spot" is typically around 2500 RPM (i.e. truck like).
It is my understanding that car engines operate most efficiently usually somewhere between 3000 and 4000 rpms
But that’s (generally) the range of peak torque and, in the real world, has nothing to do with peak fuel mileage.
The rpm of Peak Torque is the most efficient ***breathing*** of an engine. This means that you’re getting the most power for the amount of fuel used. Also, the RPM at which peak torque is acheived varies wildly from vehicle-type to vehicle-type. Peak torque on a Honda Civic Si occurs at an engine speed that would cause a V-8 Mustang to vomit its internal parts through the top of your hood. But to top it off, the RPM range of peak torque is the direct enemy of fuel mileage. Why? Aerodynamics!
For this reason, on a manual-transmission car, the highest fuel mileage will occur at (pardon my caps) THE LOWEST SPEED THE CAR WILL RUN IN ITS HIGHEST GEAR. This can be proved empirically, and will not vary.
The Honda Insight has the On-Board-Milage-Computer-To-End-All-Mileage-Computers, and I’ve spent more hours than you’d believe in testing with this car. The best AVERAGE mileage I ever acheived on my regular "real world" 100-mile loop was 113.7. I can show a 120-MPG average for most of the trip, but hills and traffic lights stop the show on one end of the loop. This mileage is acheived by driving 29 miles-per-hour in 5th (the highest) gear. This is the speed at which the Insight is idling in 5th gear. Yes, it’s dangerous to try this on most highways. I have a test loop where it’s safe, but in case of even moderate traffic, I speed up and abort the run. It’s not safe to drive 29mph on a road posted 55.
Hours of testing showed that increasing speed to only 40mph makes it very difficult for the Insight to AVERAGE much above high-80s mpg. To show yourself how powerful the aerodynamic force is at 40mph, drive your car up to 40 and try opening the door ALL THE WAY OUT. It can be done, but only if you’re dedicated to the task! And please don’t try this if you’re the type person who falls out of cars.
The reason peak torque RPM does not yield best (or even "good") mileage is because, in top gear, most modern cars would be running (as you stated) around 80 mph, automatically killing fuel mileage from the get-go.
So, to recap, manuals will always get best fuel mileage at the lowest speed they can run while in their highest gear.
Automatics will get best fuel mileage at (pardon caps again) THE LOWEST SPEED AT WHICH THE TRANSMISSION WILL SHIFT INTO "HIGH," AND THE TORQUE CONVERTER WILL STAY LOCKED UP. On my wife’s Dodge Intrepid, this speed is around 40mph, but many cars are closer to 50mph.
To non-gearheads, torque converter lock-up "feels" like another gear has shifted, so don’t confuse the two. To check the feel of TC lockup, get the car up to temperature and up to about 55-50mph. Then with a LIGHT throttle setting (but with SOME load on the engine), press the brake pedal SLIGHTLY, only enough to activate the brake lights. When the brake lights activate, this tells the ECU to unlock the torque converter. If the converter was locked-up, you’ll feel the engine speed up a few-hundred rpms.
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