A tale of two Kias: Why more powerful engines sometimes don't sacrifice mpg

Generally, if you want better gas mileage, buying a car with a smaller engine is a good idea. But it isn't always that simple, as evidenced by two Kia Sorentos we recently tested.

We bought two 2011 Sorentos: an EX with the 3.5-liter V6 engine and an LX with the 2.4-liter four cylinder, both AWD. In our fuel economy tests, they both achieved 20 mpg overall. Break it down, and they both tied at 14 mpg city and 26 mpg highway.

In principle smaller engines should be more efficient. So what's happening here?

In our testing, we've found that sometimes small engines in heavy vehicles have to work so much harder that they negate the fuel savings a consumer might expect from the smaller displacement.

In this case, the Sorento LX weighs 3,844 pounds and produces 175 horsepower—that's 22 pounds per horsepower. In contrast, the EX V6 weighs 4,060 pounds and puts out 276 horsepower, meaning each horsepower is moving just 15 pounds.

But this is just part of the story.

There are significant differences in torque, the twisting force the engines produce. This is the readily available power you feel at low revs. The four-cylinder engine produces 169 lbs.-ft. of torque at 3,750 rpm, whereas the larger engine produces 248 lbs.-ft at 5,000 rpm. Differences in engine design and components can also play a significant role in fuel economy disparities.

With any vehicle, gearing plays a significant role in how power is delivered. While both versions share a six-speed automatic transmission, the first three gear ratios differ slightly and the final drive ratio is much lower for the four cylinder. The transmission seeks to make the most of the Four's output, but there is simply less grunt to put to work. And that can be witnessed in more-frequent downshifts when climbing hills, passing, or just accelerating than with the V6.

There can be more factors at play, but the basic lesson here is that the four-cylinder has to work harder to overcome the vehicle mass and aerodynamic drag than the V6.

Mileage may vary

While the numbers are stacked against it, I found that in the right circumstances, it is possible to get better mileage out of the four-cylinder Sorento. On my longish, modest speed rural-highway commute, I observed more than 25 mpg in the four-cylinder version, versus just 21 in the V6. Of course, results may vary depending on routes, speed, and driving style.

Based on our formal testing, we got near-matching fuel economy and consequently calculate similar annual fuel cost: $1,685 for the four cylinder, versus $1,720 for the V6. That $35 a year difference won't amount to much though, compared with the higher $5,800 purchase price difference. ($26,590 versus $32,390).

The Sorento is the only new car in our current test that defies conventional wisdom. The next closest model I could find is the Toyota RAV4, which gets 22 mpg overall with the V6 engine, and 23 mpg with the four cylinder.

Our tested 2010 Hyundai Santa Fe also got the same fuel economy as the Sorento for the four-cylinder and V6 engines. The EPA gives better numbers (2 mpg each for city and highway) for the four cylinder than the V6. It could be under certain driving conditions the four cylinder will give better fuel economy, but it didn't show up in our test protocol.

We'll continue to explore this issue when we test four-cylinder Buick LaCrosse and Toyota Sienna, comparing their performance against six-cylinder versions. Expect more challenges to your conventional powertrain wisdom as the trend toward smaller engines continues. And as always, check the test results before making assumptions.