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[EHart6]

Reach YES/NO, Maintain NO. (automatics)

The low end torque on the v6 allows the speed to come up (shifting as it goes) with little or no pedal. The lack of low end torque in the I4 requires the use of additional fuel (over idle) to get up to speed (more pedal). Maintaining speed is completely different. Now the V6 is using more fuel, generating more power than needed, to stay a given speed. The I4 is closer to its max generation thus using less fuel by not generating the extra unused power. However the difference in engine sizes offset the additional pedal use in the I4. So depending on habits (as in mine) it is a break even in mpg. It is quite possible there is a V6 vehicle that can beat a small I4 for city mpg. A 3.8 liter is a fairly good size V6. A 2.8 or 3.0 might do much better. Many factors come in to play (i.e. torque, size, gearsets, weight, resistance, etc.)

Example (my vehicles);

Reach (accelerate moderately, level ground, and no headwind)
The V6 requires ~18 HP to accelerate the van. At 800 rpm it generates (~140 ft lbs) roughly 23 HP. Thus the van moves quite easily at idle (no pedal) and will even shift gears. The I4 requires ~12 HP to accelerate the car. At 800 rpm it generates (~65 ft lbs) 10 HP (One reason the I4 needs to idle at 1000 rpm, to get 12 HP). It also must exceed the 12 HP to shift (i.e. 1400 rpm or ~17 HP). To match the V6 torque/weight/resistance/accelerate ratio; the I4 must turn ~1800 RPM (requiring more fuel as you press the pedal). The more times you accelerate, the more fuel you are using. Keep in mind though engine sizes offset the additional fuel in my case.

Maintain (60 mph, level ground, and no headwind)
V6 3.8 in overdrive turns ~2300 rpm. At ~180 ft lbs torque that equates to 79 HP. The V6 requires ~28 HP to maintain 60 mph. A difference of ~51 HP or only about 35% of the power generated being used (however this leaves plenty of room for headwinds and inclines). The I4 1.5 on the other hand, in overdrive turns ~2500 rpm. At ~68 ft lbs torque that equates to ~34 HP. The I4 requires ~22 HP to maintain 60 mph. A difference of ~12 HP or ~60% of the power generated being used (doesn’t leave much room for head wind and inclines though). So it beats out the V6 in maintaining Highway speed using more of the generated power (by percentage) and shows up as better MPG by not producing the extra HP (fuel has a fixed amount of energy, how, and when, you use it makes the difference).

This is also why most V6 and V8’s (I have driven) will have their actual C / H mpg within 5-7 miles of each other (My Windstar V6, 22 City / 27 Highway, My F250 V8, 11 City / 17 Highway). The I4’s generally having a 10+ gap (My Tercel I4, 22 City / 37 Highway, and My 05 Corolla I4, 30 City / 41 Highway). The Corolla is a VVT 1.8 130 hp (better low end torque). I love that car……..

Keep in mind this is actual experience based on my driving habits and not published data. How you use you “foot” will greatly affect these numbers. In my case, mostly highway, I can sacrifice the low end torque for better use of power generated at highway speeds.