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The smaller engines don't get better mileage in normal driving patters due to being underpowered. I've compared my Jeep with a 5.9L V8 to another with a 4.0L I6. I drove the 4.0 for about a month in my normal driving pattern. Since getting mine with the 5.9, and adjusting my driving habits to the higher powered, but very similar vehicle, I am getting within 0.2 - 0.3 mpg of the 4.0 in my normal pattern of short trips and city driving. My engine has 2 more cylinders and almost 50% more displacement. The Jeeps are very similar in weight. My tires are about an inch shorter (225/70/16 vs 245/70/16), axle gearing is the same (3.73). Trans gearing is slightly different. Mine is (1st - 2.45, 2nd - 1.45, 3rd - 1.00, 4th - 0.69). The 4.0 is (1st - 2.74, 2nd - 1.54, 3rd - 1.00, 4th - 0.69).
The bigger engine allows me to gain speed MUCH faster while keeping the engine in its efficiency band. The only problem is, in heavy downtown traffic, everyone else picks up too slow in their little 4 cylinder cars. Optimal MPG for me comes with letting the auto shift at 2100 - 2200, which allows moderate throttle. When stuck in heavy city traffic, I've seen it shift as low as 1400 rpm picking up from a stop on flat ground. At that point, I'm on the gas pedal so lightly, that I can't press it any less without coasting.
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