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Pittdawg
Dodge Dakota
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10/14/2002
20:12:36
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 Subject: how do I bring rpms down and mpg up?
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I have an 02 4.7 liter auto QC with 3.92's and a snugtop, with those gears and all that weight I get really crappy mileage, I average about 13 mpg, and that's with a fair amount of highway driving mixed in. I went with the 3.92's thinking it was not going to affect my mileage that poorly but it really does. I'm willing to give up a little off the line accleration for some better mpg. I have heard when people go with a catback exhaust on the 4.7 they lose a lot of low end torque, does this translate into more horsepower and a lower rpm at freeway speeds, or is it simply a loss in power with no real benefit?
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New Dakster
Dodge Dakota
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10/15/2002
04:46:06
| RE: how do I bring rpms down and mpg up?
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Pittdawg,
The exhaust will have no effect on your gearing, thus no change in RPM no matter what you do to the exhaust.
Changing the rear-end gearing is not very cost effective...you could try some of the mods that help your air-pump be more efficient (remember, your engine is nothing more than an air pump. The more efficiently it moves air in and out, the more fuel efficient it will be, granted your driving habits don't change)
A homebrew or aftermarket intake with a K&N air filter is a good start, but really only good for about .5 MPG. Another milage improvement can be gained by installing headers and a free flowing exhuast, however, when you add up the costs of both of these mods, you can buy about 637 gallons of gas. (estimating about 200 for the intake and 750 for the headers/exhaust)
Most people perform mods for HORSEPOWER. If gas milage is a concern, find an early 90's Honda Civic for your commute.
Hope this helps!
Ryan
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Texas Todd
Dodge Dakota
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10/15/2002
08:50:23
|  RE: how do I bring rpms down and mpg up?
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I've got an 01 4.7 QC, with a Leer top, 3.55 LSD rear end, BUT it's a 5spd! Maybe that's the difference. Cause even pulling my 20ft Catamaran sailboat at 80 with the a/c on, and city driving, I've never got as low as 13mpg.
Air pump huh? somewhere in that equation is the efficiency of the combustion process that pushes the air, not just an air pump.
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Chris
Dodge Dakota
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10/15/2002
09:27:56
| RE: how do I bring rpms down and mpg up?
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Larger tires will lower one and sometimes increase the otehr.... I have a 2000 CC 4.7 5-speed and usually drive well over the limit, I average 16.5 but have seen 18 on a babying type of gas fill-up...
Chris
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Pittdawg
Dodge Dakota
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10/15/2002
12:51:18
| RE: how do I bring rpms down and mpg up?
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I'm so sick of people saying, "if gas mileage is what you are concerened about you should drive a civic". Well duh! The point is I really like my Dak and am willing to give up good mileage for the other advantages over a civic that it offers, however, I would be even happier if I could get all those advantages with less of a mpg loss than I currently have. Furthermore, from mpg people have been posting witha similar setup as mine they some to be getting a lot better, I'm trying to figure what I could do to improve mileage however incremental that may be. I understand doing modifications to improve mileage will take years to pay for themselves but hey I'm also paying for more horsepower which is always an added bonus
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Canucker
Dodge Dakota
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10/15/2002
13:23:32
| RE: how do I bring rpms down and mpg up?
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Here's my advice if mileage is really what you're after:
HO cams ... do NOT get the HO intake
3.55 gears
5spd (I know, not a realistic 'mod')
K&N filter & homebrew intake
port throttle body (either buy a ported one or port your yourself with a dremel tool)
headers ... maybe
high flow exhaust and high flow cat .. but keep the diameter smaller .. 2.5" max...
synthetic oil, diff and tranny fluid
maybe larger diameter rims and tires IF they are lighter than the stock ones
and most importantly:
a LIGHT right foot
that setup should get you pretty decent low end torque .. which is where most of your cruising is done.
hope that helps.
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Chris
Dodge Dakota
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10/15/2002
14:49:01
| RE: how do I bring rpms down and mpg up?
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Maybe it is me but I have also noticed that if I run my 4.7 around 2500-3k I get my best mileage. Granted with the 5-speed I can do this easily. Maybe take it out of OD around town?
Chris
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wjco
Dodge Dakota
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10/16/2002
10:28:32
| RE: how do I bring rpms down and mpg up?
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Take your foot off the throttle!!
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jjj
Dodge Dakota
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10/16/2002
16:10:36
|  RE: how do I bring rpms down and mpg up?
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Pittdawg,
I have a 01 QC and will GLADLY trade you my 3.55 for you 3.92's. I can do all the work - swap complete assemblies, only have 10000 miles on mine.
I open to this if you are and your rear is a posi. Don't really want to give up the posi.
Ex-DC tech so it would only take a couple of hours.
Jeff
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JMII
Dodge Dakota
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10/16/2002
19:25:19
| RE: how do I bring rpms down and mpg up?
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Thats the same mileage I'm getting while towing my boat on the highway (2,000lbs @ 70MPH) and around town/city with my '02 auto QC 4.7 w/3.92. You do know that those Snug-Tops (the full height ones) are huge drag creators. Maybe your 1st mod should be to take it off.
The only thing that pisses me off about the mileage is the 14/19 that is on the window sticker, there is NO way this truck could get 19 on the highway unless it was going like 40MPH in OD and downhill!
BTW Civics can't tow boats... thats why I got a Dak :p
- John
http://home.mindspring.com/~jntmoore/truck/home.html
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Steve
Dodge Dakota
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10/17/2002
18:40:50
| RE: how do I bring rpms down and mpg up?
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I have a 01 rc, 4.7, 5 speed, 3.92 LSD, with a flowmaster catback and i get around 18 to 18.5. thats country driving, and trying not to lay on it
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ogc
Dodge Dakota
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10/17/2002
20:02:53
| RE: how do I bring rpms down and mpg up?
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headers are almost no help at all on a 4.7 due to the design of the ex. manifold. there is only a 2.2% gain at the crank if you install headers, this is according to Mopar Engineers that designed and tested the engine. that amounts to about 3-4 hp at the rear wheels. cost effective? not hardly. I have seen others post on these forums a magazine article that said much the same thing. the easiest way to drop rpm is larger tires, either going up from say a 70 series to a 78 series with the same metric designation, say 235-70 to 235-78 will ad about 3 mph more then your speedo is actually reading at 70 mph. Not much but a start. then going to a 255-78 will help a bit more. the largest dia tires you can use IF I remember from others on here is about 31 inches. That could, depending on your original tire size, be as effective as changing the 3.90 you have to a 3.55 effective ratio, in comparason to the distance traveled per engine revolution in hi gear. The only thing is, when your speedo says 60 you might be doing 70 or more actual speed.
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dweishaar
GenIII
10/17/2002
23:31:32
|  RE: how do I bring rpms down and mpg up?
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i dunno what the problem is dawg, i get just over 14 a gallon.....wait............nope, no crying. you buy a big truck, you pay big truck prices. nature of the beast
One Eyed Fox
The Baja Dak
Have you jumped a dak..........lately?
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New Dakster
Dodge Dakota
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10/18/2002
01:44:41
| RE: how do I bring rpms down and mpg up?
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...about the civic comment that I posted...didn't mean to sound like a smart@$$. ;)
It's actually what I'm doing. I love being able to pull stuff, which is why I got my dak. I also have a 2 mile drive to work, so I'd like a beater to get there without wearing out my baby. I thought you were just going for milage, so I brought it up as a possible solution...might be cheaper to buy old commuter car than upgrading dak...HOWEVER, if you're paying for the performance too, then by all means, upgrade the DAK. Makes it easier to crush rice and rustangs. Just my opinion, which doesn't amount to much anyway. :)
Ryan
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FredDQC
Dodge Dakota
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10/18/2002
17:51:45
| RE: how do I bring rpms down and mpg up?
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Well, I found out by experience that the higher the RPM's are at your normal driving habits, the more fuel you waste. So if you usually drive at speeds between 70-80mph,and you are between 1,500-2,500 RPM's, you should be OK! Of course if you can make it happen close to the 2,000RPM's, that should be the most effective RPM for those speeds. Two things I found out to accomplish this where change to a taller(not wider)tire and decrease the gear ratio. I tried this on a ford truck by changing from 355 to 288(crown victoria).It made a big defference RPM's went down from 2,800-2,200 @ 85mph. That is almost a whole gear range.I lost take off speed but I gained cruising speed, which ended in better mpg's.
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routesixtysixer
Dodge Dakota
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10/18/2002
18:19:21
|  RE: how do I bring rpms down and mpg up?
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For what it's worth, I have a 97 CC 5.2 auto and have installed Mopar headers & y-pipe, DynoMax catback, K&N filter, MSD 8.5 mm wires, plugs, BW brass cap & rotor, electric fan (removed engine-driven fan) and mounted 245-70-16 tires (6% taller than stock) and changed out the speedo gear from a 35 to 33-tooth (to fix error). So I have effectively reduced my 3.55 gear to a 3.34. At an indicated 70 mph the tach reads right-on 2,000 rpm. When I started (bone stock with 225-70-15 tires) I got about 16.5 to 17 mpg. Now, after all these mods, I get.......... 16.5 to 17 mpg. That's right, no mileage improvement at all. Performance has improved a bit, but I really think the 44re tranny is the big culprit. Sucks horsepower/gas like nobody's business! 5-speed is really the only economical way to go in a Dodge.
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