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
91 octane: 4,551 miles, 169.73 gallons
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."
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
July 11, 2011
In May, the Cruze took honors for being the best-selling car in its segment. June sales figures reveal the little car has stepped it up a notch; last month, the Cruze rose to become the best-selling car not just in its segment, but in the nation. With 24,896 units sold, the Cruze was second only to the Ford F-150.
The fuel-efficient Cruze Eco, good for 42 miles per gallon on the highway, comprised 17 percent of the model's sales during the month of June. Retail sales for Chevrolet were up 16 percent, marking the tenth straight month of retail sales increases for the brand.
Warren Clarke, Automotive Content Editor
June 27, 2011
...not according to the pump I used to top off the tank this morning.
If we used the "4.6 gal" reported here, the fuel economy computes to 25.6 mpg. If we use what the pump reported, the fuel economy computes to 21.4 mpg.
Chief Road Test Editor, Chris Walton @ 12,025 miles
June 20, 2011
The numbers are in. Our 2011 Chevrolet Cruze has returned from Arid-zona and is back in the cooler coastal environs of SoCal.
Before we get to that, I thought I'd share a couple of highlights from a conversation I had with Mike Katerberg, the Assistant Chief Engineer of GM's Ecotec 1.4-liter turbo engine.
First off, Mike thinks you guys rock. He didn't use those exact words, but he said more than once that Inside Line has some knowledgable commenters on this blog.
He confirmed my suspicions about the intercooler for the low-speed launch behavior, but also pointed out that extreme hot weather favors the use of 91-octane premium unleaded. My use of 87-octane in these conditions led to my loss of drive-away power as well the wonky fuel economy over the trip.
In mild weather, 87-octane does just fine, but with high heat comes detonation, and the Cruze's ECU counters by retarding the timing, a move that does engine torque output and efficiency no favors.
At drive-away, where V = not much, my heat-soaked air-to-air intercooler can do little intercooling, allowing hot compressed air into the cylinders instead. The resulting low density mixture prompts the ECU to retard the timing to ward off detonation. The intercooler comes back to life with speed and airflow, but the overall high ambient temperature conditions still bring with them a higher chance of detonation. So there's still less spark advance at cruising speed in hot weather, it's just harder to tell. And you've got no way to feel it happening if you're on cruise control, as I was.
JKav, back from LeMans but still jet-lagged, is drawing me an MBT graph (maximum brake torque) graph showing how turbos benefit from the spark advance that comes with higher octane far more than naturally aspirated engines do. "Turbos like octane," he said. He's explaining the tech behind that "no duh" knowledge.
But this octane sensitivity is even more true of small engines with a small turbo like the Ecotec 1.4T engine. Such mills reside further down the inverted parabolic MBT curve from the theoretical apex point of optimum ignition timing. Anything that allows more timing, such as 91-octane fuel, will make a noticeable difference by allowing torque to march up the steep part of the curve. Jay scribbles a few tick marks with his pen saying, "A little goes a long way."
On the freeway, the cutback in ignition timing reduces engine torque and efficiency for a given amount of throttle, which means that even though I'm not aware I'm doing it I am in fact loading the engine to a higher degree by booting the throttle a little bit more to keep my cruising speed where I want it. Unlike a large-diplacement blown engine, the turbo on a small forced-air engine such as this is actually doing something other than pinwheeling while cruising on the highway, meaning it's more sensitive to the above-described ignition timing and efficiency issues.
Enough of the Dr. Science routine already. Here are the overall trip mpg numbers for the Cruze 1.4T to and from Phoenix and the Buick Regal 2.0T to Las Vegas and back.
2011 Chevrolet Cruze LTZ 1.4T automatic
- Phoenix and back, 672 miles
- 87 octane unleaded
- 36 mpg EPA highway rating
- 28.1 mpg trip average
- 111 degree peak
2011 Buick Regal CXL 2.0T automatic
- Las Vegas and back, 578 miles
- 91 octane unleaded
- 28 mpg EPA highway rating
- 30.8 mpg trip average
- 107 degree peak
The weather was more or less equally hot on both routes and both of us used cruise control. Erin apparently had a couple advantages: premium gas and a larger displacement turbo mill.
As for the Cruze, it's certified to run on 87-octane fuel because that makes sense for most buyers most of the year. But that doesn't mean that 91-octane fuel is always the more expensive choice in hot weather. The use of more expensive fuel on this trip would have paid for itself on a cost-per-mile basis if it had pushed average mpg up to just 30 mpg. Anything higher than that and I'd have saved money by buying "expensive" gas for this trip to Phoenix.
This impromptu comparison wasn't as scientific as it could have been -- Erin I basically compared notes and realized we'd driven much the same way in much the same conditions. She happened to use 91-octane because she felt like it.
Look for a repeat test with more of the variables controlled, but it seems that desert dwellers that drive certain small-displacement turbo cars may actually save money by paying for premium gas in summer.
Dan Edmunds, Director of Vehicle Testing
June 17, 2011
Day Two of my quick Phoenix excursion in our 2011 Chevrolet Cruze dawned just as bright as the day before. It was clear from the outset that this was going to be another hot one.
First thing in the morning I topped up the fuel tank to isolate yesterday's long freeway drive from today's in-town driving. In the relatively cool Phoenix morning air, the heat-related turbo boost problems I had yesterday were gone.
The run from my home in Orange County to here had been a vague attempt to duplicate the conditions of the fuel economy run I made recently in a 2011 Chevy Cruze Eco. But California and Arizona freeway speed limits are set at 70 and 75 instead of 65 mph and the prevailing speed of traffic is even higher than that. I didn't dare set the Cruze on cruise at a pokey 65 mph like I did in the Eco on the east coast. Instead I settled on a 70-mph target speed to keep things close without making myself into a rolling chicane.
Our Cruze LTZ with its 1.4T motor and 6-speed autobox is EPA-rated at 24 city and 36 mpg highway. I was able to match the Cruze Eco's 42-mpg highway figure a couple of weeks ago. Could I achieve 36 mpg in the Cruze LTZ here?
The short answer is no, not even close.
Our Cruze LTZ gulped 12.19 gallons of unleaded after 364 freeway miles on a run that included a smaller number of side trip miles than I made in the Eco. That works out to 29.9 mpg, a full 6 mpg shy of this car's EPA highway rating.
Driving at 70 instead of 65 mph probably accounts for no more than 1 or 2 of the lost mpg. So what's left? Heat. Oppressive desert heat. My schedule forced me to make the drive in middle of the day, and the outside air temperature gauge hovered between 105 and 111 for at least 275 miles of the trip. Perhaps the boggy low-speed behavior I experienced was an overt sign of a more subtle loss of turbo efficiency that blanketed the entire run.
But that's not an entirely comfortable explanation because the Cruze also did worse than Erin in the portlier Buick Regal turbo, a car rated at only 28 mpg on the highway. On a similar hot desert run at a higher cruise control set speed she managed 30.7 mpg. She managed 30.9 mpg on a return trip run at the same 70-mph target speed I used for this run. In short, her Regal is kicking my Cruze's ass.
The only other difference I haven't mentioned yet is the fuel used. Erin used 91-octane supreme in the Buick while I stuck with 87-octane regular in the Cruze, just as I had in the Eco test weeks earlier.
What conclusions can we draw from this? Not sure. I need to drive home and bake in the return trip data first.
Dan Edmunds, Director of Vehicle Testing @ 11,107 miles
June 15, 2011
Where on earth did our 2011 Chevrolet Cruze encounter 111-degree weather? About 50 miles west of Phoenix, that's where. It was well over 100 for about 250 miles of the trip, and the temperature gauge reached triple digits pretty much as soon as I dropped into the desert near Palm Springs.
I had decided to drive the Cruze on a quick business trip to Phoenix because, the way I figured it, I'd spend less time getting here by car than I would if I drove 50 miles west to LAX, parked someplace and rode a shuttle, timing that to arrive at least 1 hour early. After that I'd go through the TSA line, board my plane, fly the 1 hour to Phoenix, wait for my bag at the other end and then, finally, grab a cab and ride 40 miles west to my hotel. No way, Jose.
So there I was rolling along I-10 on cruise control at 70 to 75 mph with no trouble, listening to podcasts and playing with the fuel economy meter to pass the time. Just before this picture was taken I decided to take a scenic back road and got off the I-10 freeway for a bit. Shortly thereafter I stopped by the side of the road for a few minutes to stretch and take a few pictures.
Through all this the Cruze sat parked on blacktop, about 10 minutes, I'd say, with the engine off. When I got back in and drove away, or tried to, it started normally but then acted as if the parking brake was still on partway -- except it wasn't. Not much acceleration, weak throttle response, and no warning lamps or any overt signs of trouble, either.
JKav isn't back from his LeMans trip, and since I'm not the engine guy around here I can only take a stab at it. I'm thinking the intercooler was heat-soaked because it felt like the turbo was pretty much 100% out of the picture. The acceleration was just about as bad as you'd expect from a normally aspirated 1.4-liter four in a car of this size. If I had had a VBOX on me (sorry, fresh out) I'd bet that the 0-60 time was no better than 13 or 14 seconds, probably more. And it wasn't smooth about it, either.
I repeated the pattern of driving followed by a 5- or 10-minute stop on asphalt a couple of times and it happened over and over. Once up to speed and cruising again, everything was fine and the brisk acceleration this mill is capable of returned.
I'm not sure if this is normal hot-weather behavior for a 1.4T Cruze. I'm here another day and I'll be keeping close tabs on it. Tomorrow morning should be cool and I'm expecting no such behavior unless and until things heat up later on.
Dan Edmunds, Director of Vehicle Testing @ 11,212 miles
June 02, 2011
General Motors conducted an informal fuel-economy test recently, with two Chevrolet Cruzes as the test vehicles. While not as extensive as our Fuel Sipper Smackdown, this test did highlight how different driving styles can affect a car's fuel economy.
GM fuel-economy engineers Ann Wenzlick and Beth Nunning drove identical Chevrolet Cruze LTs. The car has EPA ratings of 24 mpg city and 36 mpg highway. They drove the cars on a 20-minute route that included city and highway driving as a stop for coffee. Their results showed that taking care of small details can potentially save drivers as much as $100 a month at the pump.
April 01, 2011
Coincidental to our monthly fleet fuel update, I topped off the Cruz and was more than a little gobsmacked at the total cost. I know we're all waiting for the Raptor to break a Benjamin at the pump, but seeing over $50 of gasoline go into the Cruz was a bit of a milestone in itself.
Chief Road Test Editor, Chris Walton @ 8,120 miles
February 01, 2011
As I'm sure you already know, our 2011 Chevrolet Cruze went in for a little servicing last week. And though I can promise you the freshly lubed strut assembly and shiny new label have done wonders for the car, it's the transmission reflash that you're probably interested in.
The last time I drove our Cruze the transmission felt like, well it felt like I built it. In wood shop. But this time, fresh from the dealer, the car had none of the old surge-y indecisive nonsense it had before. Instead, the Cruze progessed through its gears like any automatic does these days. The only thing I noted was the transmission's tendency to hang on to second gear (past 4k rpm in normal driving conditions) most every time.
I won't speculate as to how the Cruze came to leave the factory with such a terrible calibration, but I am glad that the solution was a quick and easy fix.
Kurt Niebuhr, Photo Editor @ 5,792 miles
January 25, 2011
I'd planned to fuel up our 2011 Chevrolet Cruze LTZ before my quick trip to San Diego, but there was a line at my usual gas station, and I got impatient, and hopped on the freeway, figuring I'd fill up somewhere in Orange County. The "distance to empty" meter promised about 89 miles at that point.
But I made it all the way to Oceanside before I really needed to fuel up -- 20 miles past when the low fuel light came on. That was an easy 451 miles on that tank, and I wasn't even trying. My pace was perhaps 70-75 when the freeway was clear.
I then put in 14.067 gallons of 87 octane, which is 32 mpg. Not an earth-shattering number, but still very good, and right on target with the 24 city/36 highway EPA rating on the automatic-equipped Cruze LTZ.
What I really like about this car, though, is its big gas tank -- 15.6 gallons -- meaning that a 500-mile tank should be in the cards for us without too much effort.
In reviewing the specs for the Cruze Eco model (which comes with a taller-geared version of the 6-speed manual gearbox), I noticed this car only has a 12.6-gallon tank. Of course, the Eco has a higher 28/42 EPA rating, but in theory, if the Cruze LTZ and Cruze Eco both hit their EPA highway targets, our LTZ would still have a longer range due to its bigger tank.
More notes from the trip coming tomorrow.
P.S. The photo was obviously taken when I was fueling up, accounting for why the fuel gauge needle points almost to the full mark. It was pegged near the E when I pulled into the station.
Erin Riches, Senior Editor @ 5,461 miles
December 24, 2010
Coasting sounds so great.
It makes you think of coasting down a long hill on a bicycle that first time when you were a kid. All that speed, even while you were saving up energy for the next uphill. Coasting is wonderful.
Except in our 2011 Chevrolet Cruze LTZ. It wants to coast all the time to save energy, only it's getting in the way of our pedaling.
It's easy to understand Chevy's obsession with minimizing even the slightest bit of frictional losses here, since not only is there an impact on the frequency of your personal visits to the gas station but also an impact on the overall fuel economy of the entire fleet of Chevrolet vehicles. That is to say, since the Chevy Cruze gets really good (and boring) mpg, the Chevy Corvette can get really bad (and fun) mpg.
The trouble is, the torque convertor of the Cruze's Hydra-matic 6T40 seems have been tuned by Chevy to slip into the coasting mode every time you let up on the gas even a fraction. And then when you get back on the throttle pedal again, there's an annoying delay before the driveline hooks up again - a delay accentuated by this turbocharged 1.4-liter inline-4's need to spool up to full power. So what you get is a confusing surge through the powertrain in commute traffic as you alternately ease into the gas and then lift off.
Sure, I like coasting, but driving the Chevy Cruze in traffic is like trying to pedal your bike with way too much slack in the chain.
Michael Jordan, Executive Editor, Edmunds.com