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  #1  
Old 10-13-2011, 02:11 AM
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2200 rpm at 65mph?

Not really sure where to put this. I have a 98 Dakota 4wd 5.2 and at 65 mph I run at 1800 rpm and get about 17 mpg highway. My wife has a 98 Durango 4wd 5.2 and at 65 mph she runs at 2200 rpm and gets about 14 mpg highway. Is this normal? Shouldnt the same motor run at the same rpm traveling at the same speed? I know the Durango is heavier. The only difference is I have a throttle body spacer on the Dakota.
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  #2  
Old 10-13-2011, 02:26 AM
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It's possible the Durango has 3.92 gears and your Dakota has 3.55s. It's also possible the tach is inaccurate, I know mine is. Even two identical vehicles can get different mileage, that's just the way it is.
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Old 10-13-2011, 02:57 AM
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Honestly I didnt even think of the gears. How can I tell what gears are in each truck. Didnt think a TB spacer would really make that big of a difference.
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Old 10-13-2011, 03:06 AM
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Honestly I didnt even think of the gears. How can I tell what gears are in each truck. Didnt think a TB spacer would really make that big of a difference.
Check the tags in the glovebox. If they aren't there any more, look at the rear end bolts. Usually one of them will have a tag that identifies the ratio.
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Old 10-13-2011, 03:06 AM
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there is also a difference in the amount of weight the same 318 is moving. the dakota is lighter is probably why its getting better mileage. the RPM difference is either rear end gears or possible tire size
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Old 10-13-2011, 03:14 AM
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Just looked and the Dakota is running 3.55 gears. The tag was gone from the Durangos glove box and I cant get under the truck right now. I should probably put the 3.55s in the Durango then. Thanks for the quick replys.
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Old 10-13-2011, 03:56 AM
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I'm pretty sure I have 3.92 gears in my 01 Durango 4.7

I have the factory tow package so I'm pretty sure the 3.92 are standard.

Going up to Austin, I got 18.4 mpg and on my way back, I got 16.2 mpg. I was going against the wind on the way back. Both times I had the truck locked in at 71mph cruse control.

I have oversized tires (265/75/16) and also the 02 JEEP TCM mod. Before my Durango ran at 2k RPM @ 65mph

Now it runs at 1600 RPM @ 65mph with the JEEP TCM mod.
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Old 10-13-2011, 06:05 AM
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Wow 1600 rpm thats low. Any improvement in mpgs?
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  #9  
Old 10-13-2011, 06:30 AM
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NO idea. The TCM was done before any road trips. I just use the truck mostly to go to/from work so I fill it up every two weeks and I get 12.4 mpg.
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Old 10-13-2011, 07:28 AM
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Just wondering, why put the 3.55's in the Durango? Usually the 3.92's will get slightly better mileage than the 3.55's if mpg is your concern. It's not all about rpm, the 3.92's are less work for the engine even though you might run a couple hundred rpm more. Plus durango is heavier, it needs all the help the motor can get.
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Old 10-14-2011, 04:50 AM
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Wow 1600 rpm thats low. Any improvement in mpgs?
Thats about right for that MPH with 3.55s, ~31" tall tires and the .67 OD of the 545RFEs 5th gear. I think my brothers truck pulls down 1,500 rpm @ 60 and roughly 1,900 rpm @ 80 mph. Crusing from Fort Worth to San Antonio at 75 mph with the cruise control set, the truck managed to get 24 mpg. At the time it had HO Cams, JBA headers, and Superchips 91 tuning.
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Old 10-14-2011, 04:53 AM
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Thats about right for that MPH with 3.55s, ~31" tall tires and the .67 OD of the 545RFEs 5th gear. I think my brothers truck pulls down 1,500 rpm @ 60 and roughly 1,900 rpm @ 80 mph. Crusing from Fort Worth to San Antonio at 75 mph with the cruise control set, the truck managed to get 24 mpg. At the time it had HO Cams, JBA headers, and Superchips 91 tuning.
Wow wish I got that!

I'm getting a CAI and a flow master 40 series. Help her breath a little better.
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Old 10-14-2011, 05:14 AM
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Wow wish I got that!

I'm getting a CAI and a flow master 40 series. Help her breath a little better.
It got nearly 15 mpg pulling 5,000 + lbs on the return trip @ 60 mph running 2,400 rpm with the OD off and we weren't taking it easy on the acceleration out of hill country (Canyon Lake).

I wouldn't put a flowmaster on it at all, get a MAGNAFLOW and a magnaflow cat installed along with a Magnaflow Y-pipe. The stock Y-pipe SUCKS, felt like we added 40 HP when we ditched it on the 03.
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  #14  
Old 10-14-2011, 05:53 AM
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I guess Im just surprised that the Durango only gets 14mpg highway when I was expecting closer to 17. I didnt buy the Durango thinking I would get good mpg but I did think it would be better than this. The only reason I would think about3.55s in it is to make it more like my Dakota since it gets better mpgs. I looked into the GC TCM and I have 98 and a 5.2 not a 4.7 so I wouldnt be able to do this mod right?
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  #15  
Old 10-14-2011, 07:40 AM
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If it has a fan clutch, check it to make sure it isn't seized. Mine was on my dakota and it was costing me 4-5mpg and making the trans shift high (loading engine).

You can go with a taller tire to knock down the rpms further if you have the clearance, but this will throw out the speedometer. If it has a limited slip, you can count how many times the driveshaft spins per 1 wheel revolution. I have had trouble doing it this way on open diffs though.

How many bolts are in the axle cover?
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