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I have 1998 Dodge Durango. It's a 4x4 5.9L v8. I'm looking at replacing the transmission and seeing as I don't really need 4wd in the city, I'd like to get better fuel economy and better acceleration.

The question is whether or not it's cost effective/good idea to turn this pulling machine into more of a muscle car machine by swapping the tranny and rear differential?

  • Are you replacing the tranny and differential because it has failed? Or are you going to replace it in an attempt to improve fuel economy? – mikes Jul 31 '15 at 17:23
  • It is old but hasn't failed me yet. I'm looking into replacing it early to improve fuel economy. Just trying to get educated :) – Andrew Graham Jul 31 '15 at 17:27
  • Are considering trying to convert it to rear wheel drive only? – mikes Jul 31 '15 at 21:32
  • Yes. But not if I don't have to. I'd really just like to raise the gear ratios to make it more of a highway truck and less of a stump puller. – Andrew Graham Jul 31 '15 at 22:56
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    Unfortunately, a large V8 and 4500 pounds can never do well for economy figures. You would need a serious weight loss to make it into muscle car acceleration territory - and those vehicles are not known for stellar thirst either. Perhaps it would best to leave it as it is, and purchase something smaller to run about town. – ALAN WARD Aug 1 '15 at 17:31
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If the transmission is slipping now a rebuild will give you a slight gain in mpg. If you have the optional towing differential with a 3.92 ratio then changing to the standard 3.55 will also yield a small gain. This will hurt acceleration though, while increasing highway mileage. The biggest issue is "cost to benefit" ratio and the time to recoup those costs that are strictly to increase mileage. The Durango is a heavy vehicle with a large motor. A realistic mpg would be 12-14 in city driving. If the gear change and tranny swap netted a 20% gain (this would be a huge gain) to 14-17 mpg. With a cost of $4000 (ball park estimate). At 15000 miles per year the fuel savings would be about 350 gallons per year. At $3.00 a gallon that's 4 years to get a return on investment on a 17 year old vehicle

  • You have a good point about turn around on the investment however I like to think that no vehicle is ever really dead so the actual age of the vehicle is irrelevant to me. What is relevant is the immediate investment required to up the gear ratio beyond what would be considered "stock" – Andrew Graham Aug 1 '15 at 2:52
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    I agree I drive them till the rust gets so bad it becomes unsafe. The gist of the answer is you can buy a lot of fuel for $4000. – mikes Aug 1 '15 at 10:32
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"Better fuel economy and acceleration" are typically mutually exclusive.

You'll gain one at the expense of the other.

The easiest way to influence these is by the use of your right foot. Unless you can significantly improve the power to weight ratio of the vehicle by shedding weight.

Before you go too far you might want to consider the cost of your changes and the anticipated fuel savings. In many cases it will take many years to get a return on your investment.

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