The Ultimate Hot Weather MPG Test - Regular vs. Premium - 2011 Chevrolet Cruze LTZ Long-Term Road Test

2011 Chevrolet Cruze Long Term Road Test

2011 Chevrolet Cruze LTZ: The Ultimate Hot Weather MPG Test - Regular vs. Premium

September 08, 2011


(Photo by Glenn Paulina)

Most volume-selling economy cars, including our 2011 Chevrolet Cruze LTZ, are certified on 87 octane regular unleaded because, well, it costs less at the pump. But many of the same cars also carry advisories in their owners manuals that suggest the use of 91 octane premium unleaded "for best performance."

To most of us performance equates to acceleration and speed, not mpg. After all, the sort of driving that leads to good fuel economy isn't exactly taxing one's engine. Why buy premium when you're not running hard, right?

This theory began to show cracks (for the Cruze's 1.4-liter turbo engine, at least) back in June when I drove it sedately to Phoenix at the beginning of summer in an attempt to meet or beat the EPA ratings -- and fell far short.

Instead, the hot weather led to a very noticeable lack of drive-away power and sub-par highway fuel economy. Subsequent discussions with GM powertrain bigwigs and our own Jay Kavanagh revealed that small turbo engines are especially octane sensitive, which means their computers may agressively dial back the engine calibration in order to ward off knock in high load situations or in very hot weather.

To put it another way, our Cruze LTZ might've achieved better highway mpg if I had filled it with 91-octane premium unleaded for the trip to Phoenix and back.

We decided a deeper dive was in order, so we decided to subject the Cruze to an extended hot-weather MPG test. We sent the car out into Death Valley, where it spent an entire month sipping nothing but 87-octane regular, followed by another on the "good stuff", premium unleaded rated at 91 octane.

The results are surprising.

87 octane: 4,381 miles, 179.00 gallons

24.5 mpg

91 octane: 4,551 miles, 169.73 gallons

26.8 mpg

Fuel economy during the month spent on premium fuel was up by 2.3 mpg -- a gain of nearly 10 percent. Yes, the Cruze's EPA combined rating is 28 mpg (on regular gas), but this driving pattern included extensive use of the air conditioner and much hotter weather than any EPA test dyno has ever seen. The point in this case wasn't to hit the EPA rating for the Cruze; we were interested in fuel related differences.

"So what?" you say. "Premium is more expensive."

Is it?

87 octane: $3.60 per gallon (average), $645.01 for 4,381 miles

14.72 cents per mile

91 octane: $3.82 per gallon (average), $648.74 for 4,551 miles

14.25 cents per mile

Turns out that the MPG benefit was large enough that it offset the higher per-gallon cost.

We can't say this applies to every car in every climate, and the per-mile costs will of course depend on the relative prices of the two fuels in your area. But if you live in a very hot place and drive a Cruze with the 1.4T engine, you'll very likely get better MPG if you use 91 octane premium. And it's quite possible that the gains will be large enough to offset the higher per-gallon cost and save you money in the long run. It's certainly worth trying for a couple of tanks.

As for us, we're going to continue to pay attention to the issue in a variety of cars in more temperate conditions.

Dan Edmunds, Director of Vehicle Testing

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