Gas Mileage: Keep it in Perspective — and Read the Fine Print

If you've been watching car commercials lately, you've probably noticed ads claiming ever-higher gas mileage: Honda has a 29-mpg V-6 Accord; Ford touts a 34-mpg Fusion; Chevy builds a 37-mpg Cobalt. While those numbers are impressive, they often don't tell the whole story.

For starters, most ads tout the EPA's estimated mileage for highway driving. Unless you commute on lightly trafficked highways, you probably won't see that sort of mileage regularly. That's not the only thing to watch out for. Automakers are creating higher-mileage trim levels, but the extra mpg doesn't come payoff-free, and those variants can often be hard to find.

Keep the Numbers in Perspective

Advertised mpg figures usually cite the EPA's estimate for highway mileage, but the EPA also calculates combined mileage — its estimate of the sort of long-term gas mileage a typical driver can expect to get. It's supposed to reflect the way people actually use their car in the real world.

The EPA sent us its equation for determining combined gas mileage. We'll save you the exact math, but suffice it to say that city mileage hurts the combined figure more than highway mileage helps: Conventional wisdom would say that a 50/50 (city/highway) distribution would result in 25 mpg overall for a car that's rated at 20/30 mpg (city/highway); therefore, a 55/45 percent distribution ought to drop the car's combined mileage to 24.5 mpg. That's not the case. According to the EPA's formula, that car's combined mileage would actually be 22.9 mpg.

That means a car getting 35 or 36 mpg on the highway may not always be more fuel-efficient than a competitor that gets 34 mpg on the highway when comparing combined mileage figures. Case in point: Chevrolet has an XFE version of its Cobalt that's rated at 37 highway mpg but the XFE's city rating is a low-ish 25 mpg. Compare the overall mileage to a few competitors, all with manual transmissions:

Mileage and transmissions
Cars compared below are for the 2010 model year.
CarCity/highway mpgCombined mpg
Chevrolet Cobalt XFE25/3729
Toyota Corolla27/3530
Honda Civic26/3429
Ford Focus24/3528

Surprising, no? Highway mileage is what's cited in ads, but combined mileage, in most cases, is what matters more. That's not the only thing to remember.

Hardware Changes

That 34-mpg Fusion or the 33-mpg Toyota Camry might be difficult to find — and getting that mileage may have its own unforeseen costs.

More than ever, automakers are engineering drivetrain configurations to improve gas mileage versus other versions of the same car. Some examples:

  • The 23/34-mpg Fusion that Ford touts is actually the four-cylinder Fusion S with a six-speed automatic; its other four-cylinder trim levels are rated at 22/31 mpg. Why? Ford spokesman Alan Hall said the Fusion S has less equipment and therefore less weight; less weight raises fuel efficiency. Ford also outfits it with low-rolling-resistance tires and specifically tunes the automatic transmission's gear ratios for more efficiency.
  • The Honda Odyssey minivan offers a cylinder-deactivation feature that adds 1-2 mpg, but getting it requires you to step up to a higher trim level, which costs more.

In some cases automakers designate special trim levels for just that reason:

  • GM's XFE line — it stands for eXtra Fuel Economy — graces the Cobalt, as well as the Chevy Silverado pickup truck, Tahoe SUV and other models. Low rolling-resistance tires, specially geared drivetrains and, in some cases, aerodynamic enhancements help wring out the best mileage possible.
  • In the 2009 model year, Ford had a similar SFE trim — for Superior Fuel Economy — on its F-150 pickup that pulled many of the same tricks.

"It's something that's relatively easier to do," auto analyst Erich Merkle said. Merkle is president of Autoconomy, a consulting firm near Detroit. "Put a different rear end, different tires, tweak the aerodynamics a bit. It's not real costly to do it. It's pretty reasonable from an economic perspective, certainly less costly than hybrids."

Hard Compromises, and Hard-to-Find Models

Why don't automakers make those changes for all their cars? Because efficiency isn't free. It's easy enough to save fuel by getting the four-cylinder engine instead of the V-6 — or the V-6 instead of the V-8 — but these days, even the same engine can take on different characteristics. More efficient drivetrain tuning, from transmission gearing to final drive ratios, can take a toll on acceleration. And high-efficiency trims often leave off weight-adding options including four-wheel drive.

"There's this tradeoff going on," said James Smith, president of the Society of Automotive Engineers. "If someone wants a vehicle that's peppy, though, and this vehicle is not peppy enough, they're not going to buy it. These manufacturers are trying to build vehicles that have a broad spectrum appeal."

Broad as the spectrum may be, those fuel-efficient models you're looking for are not always easy to find. Within 100 miles of our Chicago offices, just 10 of the 250-plus 2010 Fusions for sale on Cars.com were listed as an automatic Fusion S, the version for which Ford touts 34 highway mpg. And of the 1,188 new Chevy Cobalts for sale within 50 miles of our offices, only a dozen carried the 37-highway-mpg XFE designation.

"The higher-contented model is generally the higher [buyer] mix," Hall said. "But, you know, once again there is a market for an entry-level car."

All is not Lost

On the positive side, cars are getting more fuel-efficient and more powerful at the same time — Chevrolet has a 304-hp Camaro that's rated at an impressive 29 mpg highway. In some rare circumstances, some cars offer trims that have both better performance and fuel economy. One example: The four-cylinder 2010 Chevrolet Malibu can get 33 mpg highway and accelerate quicker thanks to its six-speed automatic; the pokier four-speed automatic in the base Malibu only gets 30 mpg highway.

"They're looking for any way possible to eke [out] — even if it's a mile or two miles different — looking to get any additional mileage out of a vehicle that they can," Merkle said.

© Cars.com 9/21/09