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Discussion Starter · #1 ·
I kind of made a thread about this already but I'd like to study this more in depth.

Often discussed on GMI are things automakers are doing to increase vehicle fuel economy. Whether for CAFE or for buyers, or both, fuel economy is the thing these days. Let's go through a few of these things, and try to see if there really is a difference...

REFERENCE: fueleconomy.gov

FWD is more efficient than RWD

2008 CTS: 3.6L DI, 6 speed auto: 17/26, 20 MPG combined
2008 Malibu: 3.6L, 6 speed auto: 17/26, 20 MPG combined

An engine of similar displacement and output (direct injection is mixed in though, keep that in mind) in vehicles of somewhat the same size and weight (I haven't looked it up though) with modern automatic transmissions get THE SAME mileage! Lutz gets a FAIL for pulling this one and blaming CAFE on his troubles.

and for you Chrysler fans, and to further show this:

2004 Intrepid: 3.5L, 4 speed auto: 16/25, 19 MPG combined
(neither vehicle is listed for 2005?)
2006 Charger: 3.5L, 5 speed auto: 17/24, 20 MPG combined

The Charger's 3.5L has a little more power than the Intrepid's and has a 5 speed auto versus the 4, for the record, but still, city MPG is actually better and the difference on the freeway is negligible. I recognize that the Charger's engine is a little more enhanced/tuned.



Small, turbocharged engines versus larger, naturally aspirated engines

2008 Ford Edge, 3.5L, 6 speed auto, FWD: 16/24, 19 MPG combined
2008 Mazda CX7, turbo 2.3L, 6 speed auto, FWD: 17/23, 19 mpg combined

2008 Saab 9-3, turbo 2.0L, 5 speed auto: 19/26, 21 MPG combined
2008 Malibu, 3.6L, 6 speed auto: 17/26, 20 MPG combined
2008 Fusion, 3.0L, 6 sped auto: 18/26, 21 MPG combined

The 9-3 is a slightly smaller vehicle and has less HP than the 3.6L Malibu which I thought was a good-enough example to use again. I believe the 9-3 with the 2.0L is rated at 210 hp, Ford at 221 hp, and Chevy at 252 hp. I threw in the Ford for fun and to keep you guys from saying I'm bias



V8's use lotsa gas

Old school!

1987 Thunderbird, turbo 2.3L, 5 MT: 16/24, 19 MPG combined
1987 Thunderbird, 5.0L, 4 speed auto: 16/24, 19 MPG combined

Ok ok, so neither vehicles are produced anymore, nor are either of the engines mentioned, BUT still, both engines had similar power ratings and were used in the same vehicle with similar configurations.

2008 Lucerne: 3800, 4 speed auto: 16/25, 19 MPG combined
2008 Buick Lucerne: 4.6L (N*), 4 speed auto: 15/23, 18 MPG combined

2008 Mustang, 4.0L V6, 5 speed auto: 16/24, 19 MPG combined
2008 Mustang, 4.6L V8, 5 speed auto: 15/22, 18 MPG combined

2008 G8, 3.6L, 5 speed auto: 17/25, 20 MPG combined
2008 G8, 6.0L, 6 speed auto: 15/24, 18 mpg combined

From the data above, I'm not going to deny that fuel economy suffers if you go for the V8 option. HOWEVER, I don't feel that it's as bad of a hit as its made out to be, though of course if you've got the power, you're going to use it now and then, and that won't help your efficiency :)


Compact pickups versus full size pickups

2008 Colorado, 3.7L I5, 4 speed auto, 4x2: 16/22, 18 MPG combined
2008 Silverado, 5.3L V8, 4 speed auto, 4x2: 15/20, 17 MPG combined

I did not use the I4 in the Colorado because if you're shopping for a full size, you probably need a little more than SOME capability. Obviously mileage will be better if your activities or business can fit with a smaller truck, but is it worth it to give up THAT much towing and payload capacity for 1-2 MPG?

AND, if you're a proponent for unibody trucks:

2008 Honda Ridgeline, 3.5L V6, 5 speed auto: 15/20, 17 MPG combined
2008 Silverado, 5.3L V8, 4 speed auto: 15/20, 17 MPG combined

Again, are you willing to sacrifice the power, payload, and towing capacity? Oooops, there's no reason to :D


Smaller trucks, unibody trucks, taking away V8s, going to FWD, and little turbo motors are NOT going to provide the fuel efficiency benefits at least with what's available right now.
 

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I have been saying that FWD is not more fuel efficent than RWD, it is just cheaper to produce engine trans modules for installation in many different vehicles.
 

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Also, FWD is more space-efficient, which helps fuel economy in the sense that the cars don't have to be as physically large to provide the same interior space.
The only part of the interior that you lose is footroom for the middle seat (Due to driveshaft) and possibly a small amount of trunk space. The rest is downright negligible. If anything you lose more interior room to IRS/strut braces than to the fact that a car is RWD.
 

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Discussion Starter · #7 ·

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I kind of made a thread about this already but I'd like to study this more in depth.

Often discussed on GMI are things automakers are doing to increase vehicle fuel economy. Whether for CAFE or for buyers, or both, fuel economy is the thing these days. Let's go through a few of these things, and try to see if there really is a difference...

REFERENCE: fueleconomy.gov

FWD is more efficient than RWD

2008 CTS: 3.6L DI, 6 speed auto: 17/26, 20 MPG combined
2008 Malibu: 3.6L, 6 speed auto: 17/26, 20 MPG combined

An engine of similar displacement and output (direct injection is mixed in though, keep that in mind) in vehicles of somewhat the same size and weight (I haven't looked it up though) with modern automatic transmissions get THE SAME mileage! Lutz gets a FAIL for pulling this one and blaming CAFE on his troubles.

and for you Chrysler fans, and to further show this:

2004 Intrepid: 3.5L, 4 speed auto: 16/25, 19 MPG combined
(neither vehicle is listed for 2005?)
2006 Charger: 3.5L, 5 speed auto: 17/24, 20 MPG combined

The Charger's 3.5L has a little more power than the Intrepid's and has a 5 speed auto versus the 4, for the record, but still, city MPG is actually better and the difference on the freeway is negligible. I recognize that the Charger's engine is a little more enhanced/tuned.



Small, turbocharged engines versus larger, naturally aspirated engines

2008 Ford Edge, 3.5L, 6 speed auto, FWD: 16/24, 19 MPG combined
2008 Mazda CX7, turbo 2.3L, 6 speed auto, FWD: 17/23, 19 mpg combined

2008 Saab 9-3, turbo 2.0L, 5 speed auto: 19/26, 21 MPG combined
2008 Malibu, 3.6L, 6 speed auto: 17/26, 20 MPG combined
2008 Fusion, 3.0L, 6 sped auto: 18/26, 21 MPG combined

The 9-3 is a slightly smaller vehicle and has less HP than the 3.6L Malibu which I thought was a good-enough example to use again. I believe the 9-3 with the 2.0L is rated at 210 hp, Ford at 221 hp, and Chevy at 252 hp. I threw in the Ford for fun and to keep you guys from saying I'm bias



V8's use lotsa gas

Old school!

1987 Thunderbird, turbo 2.3L, 5 MT: 16/24, 19 MPG combined
1987 Thunderbird, 5.0L, 4 speed auto: 16/24, 19 MPG combined

Ok ok, so neither vehicles are produced anymore, nor are either of the engines mentioned, BUT still, both engines had similar power ratings and were used in the same vehicle with similar configurations.

2008 Lucerne: 3800, 4 speed auto: 16/25, 19 MPG combined
2008 Buick Lucerne: 4.6L (N*), 4 speed auto: 15/23, 18 MPG combined

2008 Mustang, 4.0L V6, 5 speed auto: 16/24, 19 MPG combined
2008 Mustang, 4.6L V8, 5 speed auto: 15/22, 18 MPG combined

2008 G8, 3.6L, 5 speed auto: 17/25, 20 MPG combined
2008 G8, 6.0L, 6 speed auto: 15/24, 18 mpg combined

From the data above, I'm not going to deny that fuel economy suffers if you go for the V8 option. HOWEVER, I don't feel that it's as bad of a hit as its made out to be, though of course if you've got the power, you're going to use it now and then, and that won't help your efficiency :)


Compact pickups versus full size pickups

2008 Colorado, 3.7L I5, 4 speed auto, 4x2: 16/22, 18 MPG combined
2008 Silverado, 5.3L V8, 4 speed auto, 4x2: 15/20, 17 MPG combined

I did not use the I4 in the Colorado because if you're shopping for a full size, you probably need a little more than SOME capability. Obviously mileage will be better if your activities or business can fit with a smaller truck, but is it worth it to give up THAT much towing and payload capacity for 1-2 MPG?

AND, if you're a proponent for unibody trucks:

2008 Honda Ridgeline, 3.5L V6, 5 speed auto: 15/20, 17 MPG combined
2008 Silverado, 5.3L V8, 4 speed auto: 15/20, 17 MPG combined

Again, are you willing to sacrifice the power, payload, and towing capacity? Oooops, there's no reason to :D


Smaller trucks, unibody trucks, taking away V8s, going to FWD, and little turbo motors are NOT going to provide the fuel efficiency benefits at least with what's available right now.

Nicely done and good accurate info. It's a good read. Add another factor into the equation that some small turbo's such as in the CX7 require premium fuel. Most modern V6s or V8s now can use RUG.
 

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Wow, I thought I posted in the first one....I remember reading it though.

Anyway, I am convinced that given the same car and same engine (with same transmission gears and final drive), with the same dimensions, and weight....but one is FWD and one is RWD......the FWD would get better fuel economy. The reason is because FWD has less powertrain loss than RWD. More powertrain loss = less hp to the ground and less efficiency.

I am an advocate for engines that need only regular over premium (unless it's a badass sports car). But keep in mind a lot of times the premium is "recommended"...it's not mandatory. I used to run regular in my Acura MDX, but when i went through mountain passes with lots of gear, I preventively filled up with premium to avoid pinging (which never occurred).
2008 Lucerne: 3800, 4 speed auto: 16/25, 19 MPG combined
2008 Buick Lucerne: 4.6L (N*), 4 speed auto: 15/23, 18 MPG combined

2008 Mustang, 4.0L V6, 5 speed auto: 16/24, 19 MPG combined
2008 Mustang, 4.6L V8, 5 speed auto: 15/22, 18 MPG combined

2008 G8, 3.6L, 5 speed auto: 17/25, 20 MPG combined
2008 G8, 6.0L, 6 speed auto: 15/24, 18 mpg combined
Where, or how did you get your "combined" figures? I thought you took the average of the city and highway (Cty + hwy / 2)? Am I wrong? If I am not, then the numbers would be:

2008 Lucerne: V6: 16/25, 20.5 MPG combined
2008 Lucerne: V8: 15/23, 19 MPG combined

2008 Mustang, V6: 16/24, 20 MPG combined
2008 Mustang, V8: 15/22, 18.5 MPG combined

2008 G8, V6, 5 speed auto: 17/25, 21 MPG combined
2008 G8, V8, 6 speed auto: 15/24, 19.5 mpg combined

And forget about the trucks! GM has mastered small block efficiency. The 5.3 V8 with DISPLACEMENT ON DEMAND (or active fuel management I think they're calling it now) offers great fuel economy vs. all the competition with no loads (like towing). That's no secret. Once you put a travel trailer behind all the 1/2 ton trucks and SUVs though, the efficiency levels out. But again GM 5.3 V8 does very well for most daily driving!
 

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Where, or how did you get your "combined" figures? I thought you took the average of the city and highway (Cty + hwy / 2)? Am I wrong?
That's the beauty of mpg, it's not linear which makes it very counter-intuitive.

Let's say you have a car that gets 10 mpg city and 100 mpg hwy. 100 miles city driving requires 10 gallons, 100 miles hwy requires 1 gallon. 200 miles / 11 gallons = 18.2 mpg.

So as odd as that sounds: 10 mpg/100 mpg = 18 mpg combined (for 50% city and 50% hwy)
 

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This is an interesting insight and a good all around read.

The thing is that people like me don't want a full size truck. For example, I use mine as a truck weekly, thats why I bought a truck. And although I love the Sierra/Silverado, I can't justify purchasing that large of a truck. I don't need a full size, I don't want to park a full size, and quite frankly when I purchased, a full size was a $7,000 premium over my truck for about the same equipment (my truck everything in it that a Canyon can, standard and optional -Onstar).

So although you are correct, the mpg is negligable, there are still reasons why people purchase a midsize with a only 242hp as opposed to a 300+hp fullsize.

Just my $.02 on the matter of purchasing reasons not just efficiency.
 

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ifcar, and baloo....thanks for the insight! I guess I never paid attention to the "combined"...I had always (and incorrectly) assumed it was the average. Most of the time, I pay particular attention to the CITY mpg anyway. With how hard I drive, I typically match the EPA city ratings. But then again, I don't have a new for 2008 vehicle with the new ratings. ;)

And Canyonaro, you bring a good point too. I wanted to mention something similar, but forgot to.
 

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Discussion Starter · #15 ·
What are the vehicles that are not exceptions?
 

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2008 Silverado 2WD
4.3L V-6 -- 15/20
5.3L V-8 -- 15/20

Ferrari
599GTB 5.9L V-12 -- 11/15
F430 4.3L V-8 -- 11/16

Now imagine if you compared a larger and smaller engine tuned to make the same amount of power. Then I think you would really have an argument that it is not the number of cylinders or displacement that makes the difference.
 

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What are the vehicles that are not exceptions?
Well, EPA figures don't tell the whole story, especially if you're comparing different models with different gearing, aerodynamics etc. You also have to consider that the potential rounding error in the EPA figures is nearly 10% for a 20 mpg vehicle.

FWD vs. RWD for example is a no-brainer. RWD adds weight, typically needs more space (longitudinal engine, rear axle) and has higher drivetrain losses because you always have an additional 90° angle between the engine and the wheels. If you build two otherwise identical cars, one with RWD and one with FWD, the FWD one will need a bit less energy, whether the EPA figures reflect that or not.
 

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Discussion Starter · #18 ·
I felt the EPA figures were the most fair for a comparison.

A RWD vehicle has a rear differential so there is a 90 degree bend in power there, but look at the orientation of a typical FWD engine and trans...there's a couple bends in there.

Maybe later I'll try figuring out the weight (though not rotational mass) difference...there's a driveshaft going back there but it doesn't weight that much, and you've got a diff but if its a IRS vehicle then you're using CV shafts you'd have up front anyway
 

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The loss in efficiency of the drivetrain is pretty minimal... especially with a manual gearbox. But yeah an RWD will usually be worse, unless you have something like a old Olds Toronado with the chain going around then the same 90 degree bend going backwards with the tranny under the side of the engine LOL

But 1 percent versus 2 percent isn't a big deal, things like weight, gearing and aerodynamics can easily be in the 10 or 20 percents...

Gearing is a major advantage to larger/more gutsy engines... the Corvette is the poster child since the mid 1980's when they put tall overdrives in them. Especially with stick, always need the stick... if you don't get the stick the OPEC will beat you with a stick ;)

Granted the Corvette is pretty crappy for fuel efficiency as a 2 passenger car (a Toyota Paseo will get more than twice the fuel economy while offering basically comparable transport), but it's totally awesome for a 350 + horsepower car.



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A RWD vehicle has a rear differential so there is a 90 degree bend in power there, but look at the orientation of a typical FWD engine and trans...there's a couple bends in there.
Nah. Crankshaft, trans input shaft, output shaft and driveshafts are all transverse. The only exceptions are funky designs like the Toronado or cars that use longitudinal engines in FWD cars (some Audis and Subarus).
 
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