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I have a 2002 dakota with a 3.55 axel ratio . I have no plans to carry or pull heavy loads at this time . I would like to know the expected MPG improvement if it where changed to lets say 3.23 or maybe the next smaller number . I would like to also know an average expected price to buy and install the new rear gear . Is it difficult to reverse the procedure if I wish later to pull a small camper. It has a 3.9 motor and auto transmission
 

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My guess is you'd be lucky to gain more than 1 mpg. If you drive 12,000 miles a year, that's a savings of 35 gallons of gas a year, or about $130 a year. Even if you did the labor yourself, it would probably still be at least a 3-4 year payback, at which point your truck will be 15 years old. I'd suggest that it's not worth it.
 

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I suspect the bill would be more than $1k if you paid someone to buy and install a new set of gears in the axle. It's a fairly complex process.
 

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DO NOT FEED THE TROLLS!
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In my opinion, the only way to do this cost-effectively is to source an entire axle, brake-to-brake, from a self-service junkyard, to swap it in yourself, and to then have someone reprogram the computer to account for the changed axle ratio for the odometer, speedometer, and torque converter lockup RPM, and even then, with the 3.9, unless the bulk of your driving is highway driving, I don't see it helping a whole lot.

My '78 Cordoba had a 2.43:1 axle in it when I bought it. It was a DOG both around town and on the freeway. I replaced it with a 3.21:1 axle and the performance much-improved, and the fuel economy really didn't get much worse.

I suspect that harder, smoother tires would help just as much as different gearing.
 

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Hi Lester, and welcome to the forum. Yeah, agree with all the posts above. You will find the problem with the mileage of even the 3.9 in a Dakota is aerodynamics, not engine rpm. You can change the rear end easiest by getting an 8.75 rear end which do have different gears that are easier to find , hav the spring perches moved accordingly, and make sure the bolt pattern (or extra spare tire carried) to match, your rpm would drop, but mileage wouldn't increase worth the amount of the cost. We just can't get good mileage in this truck no matter what we do.
 

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Tires would be the largest single improvement you could make for mileage. And a bed cover if you don't already have one.
 

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That is true about the tires, they at least are a wear item and would be paying for themselves because they are a normal wear item. The larger diameter the better, as long as they fit within the wheel well, which have quite a bit of extra room in them.
 

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Aerodynamics definitely will cost you. I get 19 mpg at 65 mph, 21 mpg at 55 mph. But RPMs are only 200 more at 65 mph.
 

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Or, the cheapest way out in regards to tires and gear ratio...check your air pressure and make sure they are all the same and bump up the pressure 3-5 lbs above your normal settings (careful to not exceed the tire manufacturers recommended maximum air pressure noted on the sidewall) your ride will be a little worse, but air is cheaper than both gears or tires.
 
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