i own a 2000 dodge dakota slt club cab 4.7 v8. i mainly do highway driving from work and back about 100 miles on the days i work. i was wanting to know the advantages and disadvantages for these gears and if they are worth it. i have never really understood transmissions that well. i am wanting to know stuff like if it will decrease acceleration or my top speed, will they not be good for off roading if i ever decide to go, things like that? any help wouldbe appreciated.

 

Before I bought my truck, I test drive a couple of others with the same motor (4.7) as the one I eventualy bought. They both felt quicker than mine. I know mine has the 3.51 gears and with all the mods I've done (CAI, Exhaust, Superchip and Fastman TB), I think its now only just a little quicker than the other two which I assume had the 4.56 gears (wish I'd found this site before I bought it!). I now feel I've got the best of both worlds, got the acceleration of the 4.56's but with the quieter cruising and economy of the 3.51's. If I did a lot of off roading I'd prefer the 4.56 option. As you said you spend most of your time on the highway, I'd stick with what you've got.

Just my thoughts on the matter, Paul.

 

factory option was either 3.55 or 3.92, so I doubt you've driven any with 4.56 gears.

4.56 is really steep for a truck not running 35" or bigger tires.

With 31" tires I'm turning around 2100rpm at 70mph. With 4.56s it would be closer to 2500rpm. I've got 3.92 gears in mine.

Your 3.55s should be spinning about 1800rpm at 70 (must be nice... I only get about 13-14mpg at 70)

With 4.56s you'd top out at a gear limited 140mph (or thereabouts), but I doubt you've got the power to get anywhere near that (I don't either).
With 3.55s you could potentially hit closer to 175, but there's definitely no way you've got the power for that.

So you'll have awesome 0-60 and 1/4mile times, you may actually gain top end due to mechanical advantage, but your fuel economy will plummet.

 

I stand corrected! As I said, I wish I'd found this forum before I started looking at Dakotas. Not that I regret my current truck, by chance, it has the AWD system which I wouldn't change for the world!

If there's a noticable differance between 3.55 and 3.92, the 4.56's must be awsome.


Paul

 

on some other posts quite a few people were saying that the 4.56 gears would actually increase your gas milage, especially if you dive highway. so that is a main reason why i am looking into these gears because i am spending about $20 a day each day that i work. i am looking for something that will give me a descent boost in the mpg.

 

Not necessarily. It depends on you OD gear. In my case I have the 545REF transmission with the .67 OD gear (5th). At 70 I am turning 2500 rpm's which puts the engine closer to it's powerband. It did not Decrease my mileage but *maybe* increased it by 1/2 mpg or so. It's hard to tell since with a loud exhaust and 4.56 gears, I can't keep my foot out of it. Understand though, going to 4.56's *may* cause issues with your driveline since it's now turning faster. My truck is butter smooth until 78 mph then I develop a horrible vibration that I believe is coming from the driveshaft hitting it's critical speed. I'm still working out the math with a truck (18 wheeler) specialty shop in Dallas.
I don't know what the Dakota's have as far as transmissions and driveshafts but trust me, contact a *REPUTABLE* rear end / drive shaft shop before doing anything. I have had absolute hell trying to use local mechanics who told me they understood how to install gears. Your backlash setting is extremely important.

 

CCM - have you invested in an intake/exhaust setup? That will help alot with HWY mpg. Also things like underdrive pulley would give you better results with less room for error and for cheaper. There are a few ways to save mpg, gears are not one of them. If anything, by the factory having 3.55 gears gives you better gas mileage then 3.92's. The only way I have heard of gears helping as far as MPG is if you already have oversized tires, which kill mpg.

 

i have a k&n drop in filter and an exhaust already. both were installed when i bought the truck so i am not sure what exhaust it is. i just figured i would get better mpg than what i am getting. i have yet to check what i am getting now. i just changed the plugs they were shot. but when i first got the truck i got about 280 miles on a tank of gas. when i get the chance i am going to locate the oxygen sensors and probably replace them since the plugs looked like they hadnt been changed since bought. abo9ut how much would the underdrive pulley add to mpg do you think and is it hard to change?

 

The underdrive pulley for just replacing the crank pulley is about 50 from www.summitracing.com. I hear it adds 2mpg overall, which is more than any other single item I have seen. I forget the looks of the 4.7L (didn't do much work to mine when I had on 04), but alot of people make a tube of sorts that goes from the grill to the airbox to pull cooler air from outside the engine compartment. That will help it get all the air it can. I did notice a big diff between the drop in K&N and full FIPK on my current 5.2L though, so maybe an upgrade there can help.

If your going for nothing more then MPG increase then you have to weigh cost of part against savings in gas. Gears, say they cost 500 in parts and labor, and save you 5 bucks a tank. You will need to fill your tank 100 times for it to just pay for itself, let alone SAVE you money. So guesstimating your driving by the 20 bucks a day, it would take a year or so to see savings. if you plan on having the truck over a year, then a possible option, if you don't plan on it, don't waste your time. (all figures are guesstimates)

I myself would go full headers on back exhaust and FIPK before gears....but thats just me.

 

That's $500 per end, so a 4x4/AWD would be $1000 to get a gear swap done, not including adding any locker/lsd.