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curious
Dodge Dakota
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7/04/2009
10:16:55
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 Subject: diff ratio for better MPGs
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I have a 2002 2WD Dakota with 4.7 auto and 3.55 rear end. I recently installed a Jeep tranmission controller and gained a lower ratio final drive from the transmission, my high speed RPMs dropped by 250-300 RPMs. I haven't done any long highway drives yet so don't know how much, if any, MPGs I've gained. I've heard folks on this site claim better MPGs with higher ratio differentials. Before changing the trans controller, I might have gotten 19 highway MPGs. Would a different rear end help MPGs??? I'm not even sure what's available, 4.10 or 3.92?? Does anyone have a similar setup, who can quote their highway MPGs?
I live in Florida, its pretty flat, no hills or mountains.
I typically do mixed highway/suburban driving, I don't load it up too often and may pull a trailer but only once a year or so.
I'm not looking for performance, just MPGs.
Besides HO cams, the rest of the engine is stock.
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Kowalski
GenIII
7/04/2009
12:49:14
| RE: diff ratio for better MPGs
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Let's clear up a common misconception here. The higher numerical ratios you mention are actually called lower ratio rear gears. Some, like myself, have seen small mpg gains going from 3.55 to the lower 4.10 rear gear. I went from 15 to almost 16 mpg with the swap on my 4x4 in mixed driving like you do. My milage is worse than yours due to 4x4, lots of hills, and probably driving habits. The higher final drive you have now might be best for pure highway driving; with mixed driving you could see a bit better with the lower ratio. But not enough to justify the cost if you're looking to recoup the $$$ in milage - this wouldn't be worthwile for you unles you are interested in the performance increase as well.
Lead, follow, or get out of the way
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HCW
Dodge Dakota
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7/10/2009
18:03:41
| RE: diff ratio for better MPGs
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Hi,
I did this mod about a year back. While I never have figured out "around town" MPG, because I figure that it will be ugly, I do notice that the truck goes around 25 to 40 miles further on the trip odometer before the low fuel light comes on.
I recently took some furniture from here in NC up to western VA. I had to go two lane blacktop in the rain all the way. My bed cover was up at about 30 degrees from horizontal, and I don't think I got over 65, mostly 50-55. I got slightly better than 23 MPG to VA. From there I went on to KY via interstate at 65-70 and got 21.4. On the way home to NC it was all interstate and I had the bed cover down. I got just over 22 for this run. I had been thru the area a number of times with the old TCM and never did better than 19.3. So I like the mod just fine.
Last year when I recommended this mod to someone a couple of folks came on and started spouting various transmission lock up figures for different gears. Their main point seemed to be that if you did the mod that the transmission would downshift directly from five to three without going to four. So you would lose all of your gas savings because the truck would rev in three more on hills. I have not found this to be the case at all. The truck easily goes down to four from five. Now if you have a big load and are trying to go up one of those 7% 7 mile grades in West Virginia at 80, yeah you are going to be in third revving hard. But you would have been with the stock 45RFE as well.
This has been a great cheap mod for me, and it should be super for Florida. The only problem I have had is that the transmission acts like it is looking for a gear when you are at very low speed and at the bottom of a steep grade, so I just manually put it in first and it works fine.
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Maxx
Dodge Dakota
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7/11/2009
09:18:10
| RE: diff ratio for better MPGs
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HCW said "I did this mod about a year back", but didn't say which mod.
Was it Curious's Jeep controller swap, or Kowalski's differential gear change?
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HCW
Dodge Dakota
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7/11/2009
10:53:18
| RE: diff ratio for better MPGs
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I was talking about the TCM. IMHO the gear swap is not worth it. The payback period would be too long. You can pick up a 3.92 complete axle for 300-500 which would make more sense to me than swapping ring and pinion gears. And since he already is running 3.55 it would not make sense in flat country anyway. If anything find a 3.20 gear, but then the shift to five would be way up there speed wise.I got the TCM for $30 so this was a no brainer. The gear swap with its down time and hassles would only be worth it for a special application. I am running a 3.55 as well.
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curious
Dodge Dakota
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7/12/2009
08:59:42
| RE: diff ratio for better MPGs
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THanks for the info
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