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Discussion Starter #1 (Edited)
Now that all the fuel economy numbers are released for the 2015 I'm really wondering what's up with the 5.7's fuel economy as it doesn't seem right with the 8 speed.

the 3.6L went from 18/27 to 19/30, an increase of 5.5%/11.1%
the 6.4L went from 14/23 to 15/25, an increase of 7.1%/8.7%
the 5.7L went from 15/25 to 16/25, an increase of 6.6%/0%

Zero percent!?

The 6.4L and the 5.7L moved to the exact same transmission yet the 6.4L saw fuel economy gains in the range what Chrysler said that transmission would get, yet the the 5.7% got no increase at all on the highway. Going by what the other engines got it should be at least 27 on the highway. something just seems fishy about the numbers, even more so when you consider the Hellcat which has close to double the 5.7L's HP is only 3mpg worse on the city and highway.

Are they purposely down-rating the 5.7L to try and get more people to go to the Scat Pack since it's basically the same fuel economy? Are the other engines all being over-rated? The numbers just don't add up the way they should.
 

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Fuel economy tests are standardized. The test may not be taking advantage of different ranges in the new transmission. For instance, my 2014 Durango RT with the 8 speed gets the same MPG from 50mph up to 75mph, because it can easily sit where it is most comfortable. You will see great MPG in a 2015 5.7l; I guarantee it, because you will drive it a variety of ways that I'm sure some angle of the EPA test doesn't cover
 

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Discussion Starter #3 (Edited)
The test being standardized is where the confusion comes in though, since a 485hp 6.4L should get worse fuel economy than a 372hp 5.7L with the exact same transmission under the exactly same driving conditions, but it doesn't, it's getting the same.

And yeah, it perfectly possible to get better fuel economy than rated, I regularly got around 26mpg on the highway on my old 05 Magnum RT which was rated at 15/23 and I've gotten as high as 40mpg on my 2013 Subaru BRZ I have right now which is rated at 25/34. I keep tossing back and forth on if I want to go with the SXT or the R/T, I really would prefer the 5.7L, I miss that engine from when I had my Magnum, but the fuel economy is one of the things that's making it harder to justify as I do put a decent amount of miles on my cars, I already have 40k on the BRZ which is just over 2 years old, if the R/T had 27-28 on the highway which was what I was expecting it would be a lot easier deciding, but there's such a huge difference in fuel economy for relatively a small increase in performance, a hit of 3/5mpg to go from 305 to 372 hp, especially considering going from 372 to 707 hp is only a 3/3 mpg hit.
 

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Just get the R/T. The A8 transmission has to be better than the numbers coming out on the R/T Challenger. Somehow manuals usually get better mileage with the old series of the R/T - but you would think the 8 speed would get 28 on the highway provided you had it mostly in cruise control and did not shoot over 75 and were closer to 65 most of the way.
 

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Chances are the EPA hwy cycle doesn't trigger the 5.7's upshifts as early to 7th/8th (o/d in ZF auto) as the 6.4's - engine load (MAP) / rpm / % of throttle are all mapped together for the TCM programming.

The 6.4 would put out more torque @ lower rpm than the 5.7 and at lower mph can upshift w/o 'bogging' down.

If the same approach were programmed into the 5.7, the transmission might hunt up/down in the gears and not really produce any real-world gains.

In many instances the factored EPA ratings are downrated from the raw mpg, and in many cases the real-world conditions can yield greater MPG than the EPA ratings (of course weather conditions, altitute and ambient temps all have an effect...).
 

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The test being standardized is where the confusion comes in though, since a 485hp 6.4L should get worse fuel economy than a 372hp 5.7L with the exact same transmission under the exactly same driving conditions, but it doesn't, it's getting the same.

And yeah, it perfectly possible to get better fuel economy than rated, I regularly got around 26mpg on the highway on my old 05 Magnum RT which was rated at 15/23 and I've gotten as high as 40mpg on my 2013 Subaru BRZ I have right now which is rated at 25/34. I keep tossing back and forth on if I want to go with the SXT or the R/T, I really would prefer the 5.7L, I miss that engine from when I had my Magnum, but the fuel economy is one of the things that's making it harder to justify as I do put a decent amount of miles on my cars, I already have 40k on the BRZ which is just over 2 years old, if the R/T had 27-28 on the highway which was what I was expecting it would be a lot easier deciding, but there's such a huge difference in fuel economy for relatively a small increase in performance, a hit of 3/5mpg to go from 305 to 372 hp, especially considering going from 372 to 707 hp is only a 3/3 mpg hit.
I have a 2014 RT and have routinely gotten 27 mpg on trips that were all freeway. This was calculated by me at the pump and not just the EVIC, and I was not babying the car so to speak.
 

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I have a 2013 Dodge Challenger RT Classic 6 speed with the 3.92 rear, city currently in Fort Rucker, Daleville, and Enterprise is 21.6. Back home Ogden Kansas, Manhattan, Junction City and Fort Rile was 22.1 - 24.7 and peaked at 25 when babying it. But of course, back home driving city to city is a nice distance drive, though I kept to the scenic routes, stayed off the highways and K-18 and chose my routes and time of day driving to avoided traffic. The worst I've seen it was 14 in the city and that was because I was a student at the university and this car really dislikes being in the city and traffic, however as soon as I got some good distance driving within in the city and was able to traverse to other towns and city (again avoiding the highway or K-18) it picked right back up in a day. This car loves and wants to be driven, the catch is, it wants to be driven not in traffic, it's sort of a lone wolf but if it has to be around a pack (or other cars like traffic) you want to be ahead of the pack.

Now for highway, I traveled about 1000 miles quite a few times now, from Ogden Kansas to Fort Rucker, then Rucker to Kansas, then Kansas to Texas, Texas to Kansas, Kansas to Rucker, Rucker to Kansas and finally Kansas to Rucker... All non-stop trips (Really only stopping for fuel and snacks). This car handles these long trips amazingly, very comfortable, 13-15 hr trips about 1000 miles one way except the Texas trip that was a 6 hr trip there then add a 6 hr back. At 75-80 mph the car gets 27-30 mpg provided there are no hills, however, there are hills so it stays consistent at 27.6 wanting to go up (considering the trip from Kansas to Rucker has 3 routes to choose from). The city driving I usually shift at 1500 rpm to 2500, if there's a hill 2000 rpm, the car handles this just fine. Highway driving doesn't matter on rpm cause I traffic is usually in a haul ass mode so I try not to be an obstruction but I try not to go above 5000 rpm when initially exiting the ramp then the shifts obviously drops to like 2500 - 3500 rpm per shift till 6th which I think settles just under 2000 at 75-80 mph (I think, can't remember, having a brain fart).

Now I have put some mods/bolt-ons on my vehicle which do aid the performance of the vehicle which does also help fuel economy as well. What I have noticed from before I modded my vehicle to where it is now, the 27-30 was achievable and more consistent due to the mods whereas before it would barely get to 27 and hold there for Highway. City wise, roughly same numbers, however, this baby is more willing and able to reach the upper numbers faster than before at stock.

The 5.7 hemi is an amazing engine... Wish at stock it would've had more horsepower and torque than what was given (especially as a competitor to the Mustang GT and the SS Camaro which are lighter vehicles with their respective 5.0 and 6.2 engines power settings). A positive to the engine in this regard is that the engine has been out for a while and thus there are plenty of mods to add to make up for this. A possible theory crafting thought is to wonder what if the current challenger weighed as much as the 1970 challenger, then imagine how much fuel economy would be, imagine the feel of power too.

I actually still have more mods to put in this vehicle, looking at getting some Edelbrock cylinder heads, wonder how much better my fuel economy will get then, or better yet, my final big mod the 400 or 410 Stroker kit. Obviously a bigger displacement will require more fuel so you could say it will drop... however with a 6 speed manual and since I don't intend using the vehicle for racing (pure daily driving but with the option to put the pedal to the metal, much like the trim implies * R/T = Road/Track, roadable being streetable and trackable being raceable*). I believe with my driving I can prove such an engine modded this way can be a great daily driver and save fuel and be fun too.
 
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