I drive a 2015 Subaru WRX Premium equipped with a COBB Stage 1 performance tune optimized for 93 octane fuel. Typically, I achieve around 28 miles per gallon through a combination of highway and city driving. This aligns with the EPA’s highway fuel efficiency estimate, notably surpassing the combined rating of 24 MPG, despite my somewhat heavy foot on the accelerator. Meanwhile, my wife relies on a standard 2016 Ford Focus SE while her Flex is undergoing significant warranty repairs. The EPA estimates the Focus should deliver a combined 31 mpg, yet it averages only 26 mpg, even falling short of its city rating. How is it that my WRX, which features a turbo, two additional drive wheels, and more than double the horsepower, can outperform the Focus in fuel efficiency?
The Focus’s automatic transmission—contrasted with my WRX’s manual—should not impede its fuel economy, as both vehicles have six-speed transmissions. The old stereotype that automatic transmissions result in lower fuel efficiency is outdated. For instance, the five-speed manual Focus is rated for a 28 mpg combined efficiency, while our rental’s automatic is expected to achieve 31 mpg. So what could be the issue?
As I noted earlier, my driving style is not exactly gentle, and neither is my wife’s. She is used to operating cars that possess three times the Focus’s 123 horsepower. Both her Flex and my WRX can accelerate swiftly with little effort, whereas the Focus lacks a turbocharger or high compression. To achieve comparable acceleration, the Focus’s engine needs to work much harder, which results in it utilizing a greater share of its maximum power compared to our turbocharged vehicles, which manage to accelerate more efficiently.
If we committed to conserving fuel, we could likely achieve even better results. Take, for example, my previous Miata, which typically returned 30 mpg on city streets. However, during a lap at Lime Rock Park, my fuel economy plummeted to 10 mpg—comparable to that of a Hummer H2. I kept the accelerator pressed down the entire track, only letting up for braking and cornering. This style of driving is anything but fuel-efficient, leading to the poor fuel mileage.
As for why my WRX manages better fuel economy even with the Stage 1 tune, it might be related to the tuning itself. Matthew Bryant from COBB Tuning support explained, “While we don’t advertise fuel economy improvements, generally, due to optimized tuning, customers can occasionally see a slight improvement in fuel mileage under average driving conditions.” This actually makes sense. The optimization process is focused on enhancing engine efficiency, whether the aim is to boost power or fuel economy. Therefore, while my primary objective with the tune is to increase performance, improved fuel efficiency might just be a positive side effect of the engine’s optimization.
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