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Organization claims a high number of drivers are unable to come close to mileage estimates

Most drivers know that when they buy a vehicle the estimates for fuel economy on the window stickers are just estimates. In the real world, driving the fuel economy can be much different. There has been more than the typical number of complaints about the fuel economy that Hyundai is claiming for its new Elantra.
Hyundai is claiming that the Elantra gets 29 mpg in the city and 40 mpg on the highway for an combined rated of 33 mpg. The problem is that according to the higher than usual number of complaints about the efficiency of the Elantra, the real world mileage is in the mid-20 mpg range. Drivers that purchased the vehicle based in large part on the efficiency claims are understandably upset by the real world figures. 
Consumer Watchdog is asking the EPA to investigate the mileage claims for the Elantra. The letter sent to the EPA read in part:
A notable exception to this rule has caught the attention of Consumer Watchdog. For the two most recent model years, Hyundai Motors has actively marketed its base models of the Elantra on their very high 29/40 MPG, and 33 MPG average, leaving a trail of disappointed drivers. An Edmunds online Town Hall discussion on the Elantra attracted scores of drivers who can't, no matter how hard they try, duplicate such numbers. One very public example of this was USA Today tech writer Jefferson Graham, whose Sept. 22 article on his new Elantra expressed his disappointment that he averaged only 22 MPG, a gap that no "break-in" period seems likely to fill.
Consumer Watchdog also pointed out that while automotive publication Motor Trend named the 2012 Elantra a Car of the Year contender, the fuel economy it achieved in testing was only 26.5 mpg. That number was poor enough compared to estimates for Motor Trend to mention it in the review. The consumer organization is asking the EPA to retest the Elantra and if it finds the estimates Hyundai is giving aren't accurate to impose a fine on the automaker to compensate drivers.
One of the big selling pints of the Elantra was that the 40 mpg highway claim was for the normal model of the car whereas other automakers needed special trims to hit 40 mpg. Hyundai also has the Accent with the same 40 mpg claim. Chevy has touted a version of its Cruze, the Eco, which gets 40 mpg on the highway. Ford has a special version of the Focus with a claimed 40 mpg highway rating that is called the SFE.
In October of this year, Hyundai announced that it planned to offer a plug-in hybrid to go against the Prius called the Elantra Touring

Source: Consumer Watchdog

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RE: Ford Focus
By Stuka on 12/5/2011 3:17:13 PM , Rating: 2
Assuming level ground, in practice, RPM is the only factor in economy. The lowest RPM you can achieve in the highest gear for the speed you want will net you the best economy. There are obvious variables, ie. wind, hills, turns, throttle position, but the only practical control point is RPM.

My DSG will hit 6th gear around 45mph which puts the revs near 1800; which just happens to be the point before the turbo starts spooling. If I remember correctly, cruising at 45mph in 6th gives an instant economy easily in the 50s.

RE: Ford Focus
By Solandri on 12/5/2011 8:51:40 PM , Rating: 1
No, RPM determines at what point in the engine's efficiency curve you're operating. If your engine operates at peak efficiency at low RPM, then low RPM will help improve mileage. If its peak efficiency is at a higher RPM, then a low RPM will actually hurt mileage.

For energy needed to move the car, it's almost entirely dependent on velocity (assuming steady state speed). You have two main sources of energy consumption:

- A steady "hotel load" below which the engine's fuel consumption cannot drop (i.e. you're still burning that much stopped at a red light). The faster you go, the less of this energy you use per distance covered. So faster is better.

- Air resistance. This goes as roughly the cube of your velocity at high speeds, so the faster you go, the more energy you use per distance covered. So slower is better.

When you add these two together, both slow and high speeds consume more fuel. 40-50 mph is about where the sum of these two yield the least energy required per distance traveled. That's considered unbearably slow for highway travel, so the speed limit was set at 55 mph when the country was trying to cut oil usage.

RE: Ford Focus
By Spuke on 12/5/2011 9:15:02 PM , Rating: 2
When you add these two together, both slow and high speeds consume more fuel. 40-50 mph is about where the sum of these two yield the least energy required per distance traveled.
Depends on the car. Mine is in 4th (5 speed) at those speeds and that's definitely not where I get my best mpg. Between 60-65 mph is where my best mpg happens but I have been able to mix it up a little (a few miles of stop and go) and still get my best mpg.

RE: Ford Focus
By steven975 on 12/6/2011 1:02:43 PM , Rating: 2
I won't rate you down for this, but your assertion of RPM being the prime factor is false. The prime determinant is really the amount of air going through the motor. More air means more gas. In many ways, MPG is determined more by fuel consumption over time. Gearing, drag, engine displacement, and engine design all affect that.

My S2000 runs 4000RPM in 6th gear at 75mph, but returns better-than-rated highway MPG of 27MPG. Doing 4000RPM constantly in most cars will overheat the oil and/or make mileage go into the 10s. Some of it is due to some power and efficiency upgrades I've made, but it met the EPA number without it.

At 4000RPM, I don't need to press the throttle as much, so less air per stroke is coming in. At lower speeds, I need to use more throttle in 6th gear, so mileage is in the same ballpark. I've tested numerous cruising speeds from 60-80 and they're pretty much the same except when I approach 80 and drop 1-2MPG.

My engine is most efficient at higher RPMs, because it has a ported head, huge valves with high lift, and a low-mass forged rotating assembly. An engine with long intake runners, small valves with low lift, and a heavier cast rotating assembly will be more efficient at lower RPMs and get good mileage at that range.

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