Ford Flex May Have Superb Aerodynamics, but Fuel Economy Is Nothing Special

By Scott Doggett July 16, 2008

FordFlexAero.jpg

UPDATES WITH FORD REACTION

Right, 2009 Ford Flex in a wind tunnel.

By Scott Doggett, Contributor

Ford Motor Co. issued a press release today, raving about the aerodynamics of its 2009 Ford Flex, a boxy compact SUV that the automaker claims outperformed the competition in wind-tunnel tests.

"At 55 mph, the Flex needs only 8.9 horsepower while the nearest competitors in the full-size crossover segment -- the GMC Acadia and Toyota Highlander -- require more than 9.3 horsepower. Moreover, the Flex's coefficient of drag is significantly better than all of its Asian competitors. Flex tests at 0.355 coefficient of drag while the competitors are at 0.375."

So says the new release. While all of that may be true, what really matters to consumers is not the coefficient of drag or the horsepower needed to make a vehicle go 55 miles per hour, but rather the fuel economy that the vehicle's aerodynamics and other factors achieve.

According to the U.S. Environmental Protection Agency, the 2-wheel-drive 2009 Flex gets an estimated 17 mpg city and 24 mpg highway, and the 4-wheel-drive version gets an estimated 16 mpg city and 22 mpg highway. These figures are slightly better than those achieved by the 2008 Ford Flex.

The EPA's estimated fuel-economy figures aren't out yet for the 2009 GMC Acadia and 2009 Toyota Highlander. But the 2008 GMC Acadia was EPA-rated at 16 mpg city and 24 mpg highway for the 2WD version and 16 mpg and 22 mpg highway for the 4WD version. Or put another way, the 2009 2WD Flex can go one 1 mile farther on the gallon of gas than the 2008 2WD Acadia, but otherwise they are spot-on equals on the mileage front.

The EPA numbers for the 2008 Toyota Highlander are 18 mpg city and 24 mpg highway for the 2WD version, and 17 mpg city and 23 mpg highway for the 4WD version.

In other words, compared to the 2009 Flex, the 2008 Highlander has slightly better city mileage estimates for both the 2WD and 4WD versions, while the highway fuel-economy estimates are identical.

If anything, the EPA fuel-economy figures for the 2009 Acadia and Highlander vehicles will better those of the previous model year. So, we were a little baffled as to why Ford issued a press release regarding the Flex's aerodynamics, knowing as the automaker must that any autowriter worth his salt would compare the vehicle's mileage with its competitors'.

Ford spokesman Jay Ward explained that when the automaker first launched the Flex, "a lot of the reaction the press gave to us was that they liked the way the thing was styled and the fact that the thing was obviously very boxy. But actually a number of people since then have gone on to say well, yeah, it's all very well and good having a great big box that's very stylish but that's got to be about as aerodynamic as a house brick.

The purpose of the press release, Ward said, was to show that Ford's engineers returned to the vehicle and "worked hard to find a way to make something that's very distinctive in terms of the way it looks, but have actually managed to hone it in such a way that we get better aerodynamics out of it that some of our closest competitors."

As for mileage, he conceded, "your point's a valid one. At the end of the day, the fuel-economy figures in that segment are very similar."

Similar, but with at least one exception: The 2009 all-wheel-drive Subaru Forester's EPA fuel-economy estimates are 20 mpg city and 26 mpg highway.

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104wb says: 1:28 PM, 07.16.08

Bravo to Ford for advertising the amount of power required to maintain a speed. The only way to know how fuel efficient your vehicle is, is to know how much work it is doing for a given amount of fuel energy used. For people who want not to be wasteful, i.e. to use the least amount of fuel energy to produce their chosen amount of work, both the power and the fuel use per unit time must be known.

Using the example above, we are given 8.9hp at 55mph or 4,895 pound-feet/sec at 80.7 feet/sec. Dividing the first by the second gives a road load of 60.6 pounds to move the Flex down the road at 55mph. In a mile (5280 feet), 319,968 lb-ft of work is done. Assuming the EPA HWY number accurately reflects 55mph condition, we know that 1/24 gallons of gas is burned in one mile, and with 116,090 BTU of energy in a gallon of gas, 4837 BTUs are consumed in that same mile. The ratio of work to energy (a unitless parameter) is the vehicle's fuel efficiency. 319,968 lb-ft = 411 BTU, so 411 BTU / 4837 BTU = 0.085, or 8.5% fuel conversion efficiency. That's actually pretty poor. The reason, I think, is because that 8.9hp figure doesn't sound right. Should be more like 20hp @ 55mph.

For those simply interested in minimizing fuel cost, yes, 'mpg' is all you need to know.
For those who think they are being fuel efficient by driving a car that gets high 'mpg', not necessarily. You could have a relatively inefficient vehicle that just isn't doing a lot of work (like a moped). On a scale of most efficient land vehicle to least, freight train is on top and moped is likely on bottom.
For those who think it is their job to dictate how much work citizens should be doing (at the office or with their car, it's all work), it's not; this is still a free country. But there's really no excuse not to maximize your efficiency (for the amount of work you CHOOSE to do, minimize the amount of fuel required to do it).

Scott Doggett says: 2:34 PM, 07.16.08

104wb, I gotta say, I couldn't agree more with the basis of what I think you're saying: We shouldn't be satisfied with just doing a good job. That, essentially, is what Ford's Jay Ward told me. The press by and large liked the Flex as it left the starting gate, but some at Ford decided the automaker could do better -- could improve upon the vehicle's aerodynamics. And the engineers managed to do just that. Hat's off to them for not only striving to do better, but actually succeeding in their efforts. More stories to write and never enough time in the day, but I wanted to respond to your comments. --gotzip

greenpony says: 10:41 AM, 07.17.08

104wb, that sounds like an analysis *I'd* do! One thing though, I doubt the EPA highway estimate is anywhere near the fuel economy at 55 mph. A constant cruise at 55? Probably gives them an increase of 20 or 30% above the EPA's number. But then you're still only at about 10.6% efficiency. With standard tires (235/60-18) and axle and 6th gear ratios of 3.16 and 0.74, I estimate 1486 rpm at 55 mph in top gear. That may not be near the maximum BSFC, given the relatively low rpm and probably small throttle opening. This could account for some of the rest of the discrepancy. Or they could have just used unrealistic conditions in their test to produce unusually low numbers..

nzvrx3 says: 11:05 AM, 07.17.08

EPA fuel economy for GMC Acadia (and Saturn Outlook and new Chevy Traverse) is 17/24 for FWD and 16/23 for AWD. And Traverse's coefficient of drag is .33.

Doesn't the Forester only seat 5?

nzvrx3 says: 11:05 AM, 07.17.08

EPA fuel economy for 2009 GMC Acadia (and Saturn Outlook and new Chevy Traverse) is 17/24 for FWD and 16/23 for AWD. And Traverse's coefficient of drag is .33.

Doesn't the Forester only seat 5?

viral says: 2:26 PM, 07.22.08

In addition, the GMC Acadia's Drag Coefficient is .34 - both it at the Traverse (.33) are bigger AND more slippery than the Ford Flex. This article is extremely misleading by claiming that it gets 'better fuel economy' (it doesn't) than its competitors because it's the 'most aerodynamic' (it is not). The GM Lambdas also dwarf it in max cargo capacity (117cf vs 83cf!) and have more horsepower and torque. They can both tow 5200 lbs vs the Flex's 4150 lbs.

To summarize - Even with all of the cards stacked against the Lambdas, given their size, they manage better drag coefficients, much more interior volume, horsepower and torque and more towing ability - all in addition to the exact same EPA fuel estimates (for FWD, AWD Lambdas beat the Flex!). Whatever formula they used to determine HP needed at 55mph, it's all smoke and mirrors, since it's doesn't seem mean squat in the real world.

Ton of Lambda info at http://www.acadiaforum.net and http://www.traverseforum.com

researchqueen says: 10:41 AM, 09.18.08

I would add that while the Acadia is a comparable vehicle, the Highlander is MUCH tighter in the third row than either the Flex or the Acadia, so if you travel with a big crew, that could be the deciding factor -- more than 1 mpg would be.

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