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  #13  
Old 08-20-2010, 09:05 PM
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Re: When can we expect better fuel mileage?

I assume when they start using the new 8-speed gas mileage will improve.
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Old 10-12-2010, 06:42 PM
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Re: When can we expect better fuel mileage?

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Originally Posted by boarderman23 View Post
I assume when they start using the new 8-speed gas mileage will improve.
Sorry to dredge up an old thread, but I doubt the 8spd will do much for economy. I test drove a Q5 with 8spd and it didn't impress me much and it's the same supplier for Jeep (ZF). If anything, I would think it could be annoying to have an 8spd constantly shifting (for minor economy gains) especially in a traction-focused environment. Since the powerbands are quite fat for the Hemi and V6, using an 8spd would only make sense, IMHO, if they made shorter 1-4 gears with "normal" 5-8 gears to give better acceleration from a stop in city driving but preserve hwy cruise; that might help the city mpg rating but do nil for highway mpg where surface area, Cd and weight matter more.

FWIW, I've asked three different WK2 owners (strangers) how their fuel economy was for their vehicle. All were V6 4x4 Laredo versions. One owner said she had no idea but could drive from Chicago to St Louis and back on one tank. The 2nd person said he didn't know and didn't care. The last person I spoke to said he got 15-18mpg (per computer) in the Chicago suburban crawl, and sees 26mpg on the computer for a long interstate trips (but not verified with brim-to-brim fill ups).
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Old 10-12-2010, 06:48 PM
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Re: When can we expect better fuel mileage?

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Originally Posted by Bmwister View Post
Sorry to dredge up an old thread, but I doubt the 8spd will do much for economy. I test drove a Q5 with 8spd and it didn't impress me much and it's the same supplier for Jeep (ZF). If anything, I would think it could be annoying to have an 8spd constantly shifting (for minor economy gains) especially in a traction-focused environment. Since the powerbands are quite fat for the Hemi and V6, using an 8spd would only make sense, IMHO, if they made shorter 1-4 gears with "normal" 5-8 gears to give better acceleration from a stop in city driving but preserve hwy cruise; that might help the city mpg rating but do nil for highway mpg where surface area, Cd and weight matter more.

FWIW, I've asked three different WK2 owners (strangers) how their fuel economy was for their vehicle. All were V6 4x4 Laredo versions. One owner said she had no idea but could drive from Chicago to St Louis and back on one tank. The 2nd person said he didn't know and didn't care. The last person I spoke to said he got 15-18mpg (per computer) in the Chicago suburban crawl, and sees 26mpg on the computer for a long interstate trips (but not verified with brim-to-brim fill ups).
Surface area, Cd and weight certainly have a more significant impact on highway mpg, but you can't discount the fact that an 8-speed will lower the highway rpms and subsequently improve fuel economy. Most manufacturers have pretty much proven that the 6, 7 and 8-speed transmissions far outshine their 5, and dare I say 4, speed counterparts in terms of size, weight and mpg.
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Old 10-12-2010, 06:52 PM
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Re: When can we expect better fuel mileage?

well, maybe this time you have luck for the new VM Diesel (around 240PS and 550nm / 405lb tork) in the US. With that engine mpg could be between 19-28mpg.
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Old 10-12-2010, 07:30 PM
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Re: When can we expect better fuel mileage?

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Originally Posted by SMG1 View Post
Surface area, Cd and weight certainly have a more significant impact on highway mpg, but you can't discount the fact that an 8-speed will lower the highway rpms and subsequently improve fuel economy.
That's too simple a statement; I hear this all the time, but reducing rpm to get better economy is only partly true if the engine can produce the required horsepower efficiently and harmlessly at that lower rpm to maintain desired cruising speed.

Ignoring acceleration: Regardless of rpm, a vehicle requires specific horsepower to maintain speed on a level road (let's just say it's 30hp@60mph for the JGC, for example)...the surface area, Cd, weight (for hill climbs) determine the horsepower needed. Add in wind and hills and the power required to maintain cruise speed goes up substantially.

You can run the engine at 6000rpm with a small throttle opening to get that 30hp or you can lug the engine at 1000rpm to get the 30hp and maintain cruising speed. Simply revving the engine at 6 times the rpm doesn't equate linearly with using 6 times the fuel, but it does have an affect...

The higher engine speed affects fuel economy from the frictional losses at the high rpm and leads to excessive engine wear. However, if you lower the engine rpm too much, not only can the engine lug inefficiently, but the low rpm may not enable sufficient oil circulation or coolant flow which could lead to engine overheating, excessive wear and early engine failure. (Idling is a minimal load condition so idling doesn't count as dangerously low rpm)

There is a median range that balances efficient engine rpm with horsepower requirements for cruising. Too high an rpm isn't good, and too low an rpm isn't good; maybe worse. It's just not simply "lowering the rpms" to get better economy, it's lowering the required power needed to accelerate and cruise; better lubricant in the engine, trans and drivetrain are also fuel economy factors among those mentioned.

Sorry to be long winded, but I've built many engines over the years and there is no changing the laws of physics.
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Old 10-12-2010, 07:34 PM
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Re: When can we expect better fuel mileage?

20 MPG Hemi compared to 14MPG in an Escalade...Im a Happy Camper
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  #19  
Old 10-12-2010, 08:15 PM
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Re: When can we expect better fuel mileage?

I have found that first of all... anything with 30" tall tires and a .67 overdrive should have 4.56's as the primary gear ratio.

RPMS alone do NOT dictate fuel economey... as mentioned above its the load on the engine at any given RPM that has more of an effect on fuel economey then RPMs alone.

With that said... Light foot syndrom can hurt your fuel economey... WHY? Because it will take you 2 miles under a constant load which will hurt your MPG.

TRICK #1

From a stop, Pull out and bring your RPMS to 2000. On shift bring it back to 2000RPMs... and do the same for each shift... untill you reach your desired speed.


TRCIK #2
Only after I have reached my desired speed using the above 2000 RPM trick I use my instant MPG readout when I accelerate. I do so in a way that my MPG stay above my current high... If I give too much throttle my MPG indicator drops like a rock and goes below my current high... but if I baby the gas I can keep the actual MPG above my current high... T Once you hit 40-45MPH let off and lightly use the throttle to bring you up to speed.

TRICK #3
COAST COAST COAST... when ever possible... Dont let your engine drag your speed down... toss it into nutral and coast to the next stop. If you know you have a stop 400-500 yards a head of you and you know you will need to use your brakes... then toss it into nutral and coast. There are a few streets where I can coast for over 1/4 mile and barely lose any speed at all before I appy the brakes to stop at the stop sign.

Other then normal maintainance like air pressure and lubircants.... Driving habbits can play a big role in your MPG.

Correcting the gear ratios will help eliminate wasted fuel on acceleration and get us into our overdrives sooner... at the same time they will offer more mechanical advantage in order to HOLD those super high ratios more effectively & effeciently.



SPEED SAFE, NICK
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  #20  
Old 10-12-2010, 09:58 PM
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Re: When can we expect better fuel mileage?

Its pretty simple physics really. You canít have horsepower and good fuel economy. Itís just not possible. There is a direct relationship between horsepower and fuel burned. The mass of the vehicle, friction, and air resistance will dictate how much horsepower is required to accelerate the vehicle to a given velocity (speed and direction).

Of course harder acceleration requires more horsepower, and therefore decreased fuel economy. Likewise, you will see better gas mileage on a highway where a constant speed is maintained for a long period of time, with not much acceleration required. Although the faster you drive on the highway, the more friction you create between your car and the air/road and consequently more horsepower is required to maintain speed, hence the usual decrease in MPG once you exceed 70 MpH or so. In City situations where lots of stopping and starting are more frequent, gas mileage suffers as a result of more frequent acceleration and less time spent traveling at a constant speed.

No matter how you look at it, a given amount of power (for example 200 horsepower) will always be required to accelerate, in this case a 5000 pound Jeep, from 0 miles per hour to 60 miles per hour in a given time frame. This is why there isnít a significant (Emphasis on ďsignificantĒ) difference between the estimated EPA fuel economies of the V6 versus the V8. Despite the difference in engine size, both will have to output 200 horsepower for example (not an actual figure, just using it for the sake of the analogy) to accelerate from 0-60. That 200 horsepower will require so many cubic centimeters of fuel to be burned to generate so many joules of energy that the mechanics of the engine can transfer to the wheels.

Albeit some engines are not as efficient as others, most built in this age are about as good as its going to get. With the introduction of engine control computers, fuel injection, and a myriad of other sensors and controls, engineers have been able to ensure that as much of the energy generated by burning fuel is transferred to the wheels as possible with as little energy lost to friction and heat as possible.

As the engine gets larger, the potential for energy grows, but in turn the amount of energy required to combat the added friction of the extra parts and simply keep the engine running grows as well. This is one of the few real reasons you will see a smaller engine get better mileage. This also applies to two wheel drive versus four wheel drive. More metal takes more energy to move, more energy requires more fuel it is as simple as that.
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  #21  
Old 10-12-2010, 10:08 PM
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Re: When can we expect better fuel mileage?

Quote:
Originally Posted by AIR_RAM View Post
I have found that first of all... anything with 30" tall tires and a .67 overdrive should have 4.56's as the primary gear ratio.

RPMS alone do NOT dictate fuel economey... as mentioned above its the load on the engine at any given RPM that has more of an effect on fuel economey then RPMs alone.

With that said... Light foot syndrom can hurt your fuel economey... WHY? Because it will take you 2 miles under a constant load which will hurt your MPG.

TRICK #1

From a stop, Pull out and bring your RPMS to 2000. On shift bring it back to 2000RPMs... and do the same for each shift... untill you reach your desired speed.


TRCIK #2
Only after I have reached my desired speed using the above 2000 RPM trick I use my instant MPG readout when I accelerate. I do so in a way that my MPG stay above my current high... If I give too much throttle my MPG indicator drops like a rock and goes below my current high... but if I baby the gas I can keep the actual MPG above my current high... T Once you hit 40-45MPH let off and lightly use the throttle to bring you up to speed.

TRICK #3
COAST COAST COAST... when ever possible... Dont let your engine drag your speed down... toss it into nutral and coast to the next stop. If you know you have a stop 400-500 yards a head of you and you know you will need to use your brakes... then toss it into nutral and coast. There are a few streets where I can coast for over 1/4 mile and barely lose any speed at all before I appy the brakes to stop at the stop sign.

Other then normal maintainance like air pressure and lubircants.... Driving habbits can play a big role in your MPG.

Correcting the gear ratios will help eliminate wasted fuel on acceleration and get us into our overdrives sooner... at the same time they will offer more mechanical advantage in order to HOLD those super high ratios more effectively & effeciently.



SPEED SAFE, NICK
You might consider that some (not all) engines of newer design actually cut fuel to the engine entirely at high speed when you release the accelerator and use the friction from the wheels turning the transmission to keep the engine turning. Then the engine is instantly reignited without the use of the starter when you press the accelerator again. However when you shift into neutral as trick number 3 suggests, the computer cannot cut fuel to the engine because the transmission is no longer turning the crankshaft to keep the engine turning and the starter would be required to reignite the engine, which would cause a lot of wear and could not be done as quickly or seamlessly. As a result you end up burning more fuel not less when you shift into neutral. Most people donít even notice when ECUs do this.
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Old 10-12-2010, 10:10 PM
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Re: When can we expect better fuel mileage?

Quote:
Originally Posted by AIR_RAM View Post

With that said... Light foot syndrom can hurt your fuel economey... WHY? Because it will take you 2 miles under a constant load which will hurt your MPG.

TRICK #1

From a stop, Pull out and bring your RPMS to 2000. On shift bring it back to 2000RPMs... and do the same for each shift... untill you reach your desired speed.

TRICK #3
COAST COAST COAST... when ever possible... Dont let your engine drag your speed down... toss it into nutral and coast to the next stop. If you know you have a stop 400-500 yards a head of you and you know you will need to use your brakes... then toss it into nutral and coast. There are a few streets where I can coast for over 1/4 mile and barely lose any speed at all before I appy the brakes to stop at the stop sign.
AHHHH 2000 RPM's? The above statement cancels that out. I don't think it is possible to accelerate any slower than 2000 RPMS, LOL

Also the neutral statement is false nowadays. You are actually burning more fuel by putting the transmission in neutral. Most cars today when you take your foot off the gas and you are coasting the fuel supply to the engine is shut off giving you nothing but the BEST mileage. When you shift into neutral your engine is using the fuel to keep itself running. In turn you are getting the worst mileage possible considering the alternative.
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  #23  
Old 10-12-2010, 10:50 PM
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Re: When can we expect better fuel mileage?

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Originally Posted by jeep2011 View Post
AHHHH 2000 RPM's? The above statement cancels that out. I don't think it is possible to accelerate any slower than 2000 RPMS, LOL

Also the neutral statement is false nowadays. You are actually burning more fuel by putting the transmission in neutral. Most cars today when you take your foot off the gas and you are coasting the fuel supply to the engine is shut off giving you nothing but the BEST mileage. When you shift into neutral your engine is using the fuel to keep itself running. In turn you are getting the worst mileage possible considering the alternative.

LOL. Same comment 2 minutes apart.
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Old 10-12-2010, 11:26 PM
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Re: When can we expect better fuel mileage?

i'm diggin your avatar ryorde. it's also a good analogy for power superceding mpg.
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