If the consumer is to believe the MPG ratings on new cars, then explain this. Two identical new minivans (same make, but EX vs. LX model) have the same engine and transmission - as disclosed on the sticker. Per Kelly Blue Book (online), both models have the same engine; same performance ratings (HP, torque, displacement, 24 valves, DOHC, etc.) Yet, the heavier model has a higher MGP rating? How is this possible, if one is to assume the MPG is factual.
Dare I WAG that the higher rating could be ploy to boost sales because the average consumer wouldn’t notice. If asked by the Feds, they can say these figures are approximate…putting the truth somewhere between the two ratings. And, who’d really know the difference when your actual MPG may vary, right?
The heavier model has a lower final drive ratio. Automakers have been tweeking the transmission and rear end ratios for years, the lower end version is a bit peppier to drive around town, the higher end version with a lower ratio is a freeway cruiser. In the real world, only those looking to buy true economy cars really worry about the listed CAFE ratings anyway. Most folks know those numbers mean nothing in the real world.
So far as I’m aware, MPG is determined by the EPA in their own testing.
The torque and HP are indicators of what you’re getting right off the engine (i.e. no car around it.) Most tests don’t determine what it is at the wheels, so there’s plenty of room to change the results. Simply changing the computer that determines the timing could give you different results, as could gearing the car different. Even if the computer is part of the engine, in this sense, it’s doubtful that they would feel it particularly necessary to change the output results per vehicle make. Just take the highest numbers you were able to get out of the engine and slap it across all makes using that engine, even if some throw 50% of its potential for zippy driving away.
Slight nitpick: The MPG is normally determined by the manufacturers themselves, using test procedures specified by the EPA. Some percentage of the results are confirmed by the EPA in their own testing, but not all. (cite)
As for the OP, yeah, what racer and Sage said. Engines are typically more efficient at higher torque points. Since mileage tests are performed at a standard vehicle speed, changing the drive ratio will change the corresponding engine speed (and torque). This could even be accomplished by changing the shift points on the automatic transmission.
A higher torque/lower speed engine operation point will typically be more efficient (*engine * efficiency we’re talking about). I would not be surprised if the added efficiency compensated for the additional weight, presuming the difference is not substantial.