3.55 vs 3.92

[bigD1031]

Thinking of buying an 03 Dodge Ram Quad Cab 4.7 and I'm currently looking at two different trucks. They are exactly the same, same tire sizes, engines, etc. but one has a 3.55 and the other a 3.92 rear end. What do you guys think, if any, kind of a difference will I see when towing a 3500lb boat? (Guestimate fully loaded with gas and gear.)

Also, food for thought, the one with the 3.55 rear end has 30,000 less miles and is about 1,000 more........

 

[SpinnerBait_Nut]

Re: 3.55 vs 3.92

Your 3.92 will have more grunt, but will also take more rpms to run the same speed as the 3.55.
Your 3.55 runs 1000 rpms at 55 mph where as the 3.92 would be turning 1500 to get the same speed.
Just an example.
I go no higher than 3.73 myself on my trucks.
Gives me a good all around number that way.

 

[dcg9381]

Re: 3.55 vs 3.92

Thinking of buying an 03 Dodge Ram Quad Cab 4.7 and I'm currently looking at two different trucks. They are exactly the same, same tire sizes, engines, etc. but one has a 3.55 and the other a 3.92 rear end. What do you guys think, if any, kind of a difference will I see when towing a 3500lb boat? (Guestimate fully loaded with gas and gear.)

Also, food for thought, the one with the 3.55 rear end has 30,000 less miles and is about 1,000 more........

It's hard to generalize about gear size due to transmission ratio differences and even more important: tire size.

I think the above poster is right though - you'll notice an acceleration difference from a stop.. Not useful for boat ramps, but useful for a ramp on to the highway.

Might check fueleconomy.gov for the practical MPG difference... It'll impact highway the most, probably little to no impact in-town.

I have a buddy with one of those trucks, I really like them - pretty darn quick. I think they're rated to tow 6k (if setup right). Should tow the heck out of a 3500 lb boat.

For grunt off the boat ramp, if they're 4wd, use low range. On my toyotas - ones with manual hubs, I leave the hubs unlocked and just use the low range for added "creep".

We're in the market for a new truck and I checked on the new Dakotas - more power, over 300, but only rated for 4k lbs... My boat/trailer are 6200.

 

[tashasdaddy]

Re: 3.55 vs 3.92

6200, you need full size vehicle, you need the wheel base, to keep from having, the wagging the dog syndrone.

 

[Silvertip]

Re: 3.55 vs 3.92

The RPM difference is not as great as was stated earlier. 3.55 to 3.92 is about 11%. Assuming the engine is running at 1400 RPM at 60 MPH with the 3.55 gears, it would be running 1554 with the 3.92 gears. As for performance, the 3.92 is better all around while the 3.55 will "generally" provide better unloaded fuel economy. Loaded however, the 3.92 may very well provide better towing fuel economy since you don't need as much throttle to maintain speed. It may also allow you tow in overdrive whereas the 3.55 might not. Yes - I know the OD/no OD arguement will spark controversy. Regardless, how you drive and the terrain you drive in determines what fuel economy you will get with either axle. If you can't drive like there is an egg under your foot fuel mileage will be poor no matter what. Your decision should also be based on how much non-towing vs towing you do. If only 10% of your miles is towing, then the 3.55's might be best. If you routinely make long trips towing in relatively hilly terrain, then I'd go with the 3.92s. Lastly, keep in mind that tire size has a bearing on engine rpm as well. If the truck with 3.92 gears has tires larger than the truck with 3.55's you may very well have close to identical RPM at 60 with either truck.

 

[bigD1031]

Re: 3.55 vs 3.92

Thanks for the input.....both of the trucks have 20" wheels with tire sizes of P275/60R20. The one with the 3.55 has after market 20" rims on it but originally came with 17" wheels and 245 or 265 70R17 and the 3.92 has factory 20" rims with the 275/60R20. My biggest concern is that if I got the truck with factory tow and 20" rims it came standard with the 3.92 and if you got the 17" rims and tow it came with a 3.55. If I go with the 3.55 will it basically be too high of gearing to be effective or comfortable towing? (I tow about 160 miles to the lake one way and it is not exactly flat driving.)

Thanks..........

 

[SpinnerBait_Nut]

Re: 3.55 vs 3.92

If it will be used also as a daily driver, I would go with the 3.55 and not use O/D when in the hilly terain.

 

[AMD Rules]

Re: 3.55 vs 3.92

I have the 3.92 rear end in my Dakota w/ factory 17" alloys, and wouldn't select that gearing again. Pig on gas, and high rpm for daily driver. Tows great though

 

[rodbolt]

Re: 3.55 vs 3.92

I have 4.10 gears and a 4.0L engine in my 06 ranger4X4.
best gas milage I can get unloaded is about 18MPG, with an 18ft hydrasports fish-N-ski its about 17MPG.
right peppy off the line unloaded though .
but here we dont have many hills so I tow with the O/D on with no problems.
gas keeps going and I will see how my bicycle tows the rig

 

[DJ]

Re: 3.55 vs 3.92

Thanks for the input.....both of the trucks have 20" wheels with tire sizes of P275/60R20. The one with the 3.55 has after market 20" rims on it but originally came with 17" wheels and 245 or 265 70R17 and the 3.92 has factory 20" rims with the 275/60R20. My biggest concern is that if I got the truck with factory tow and 20" rims it came standard with the 3.92 and if you got the 17" rims and tow it came with a 3.55. If I go with the 3.55 will it basically be too high of gearing to be effective or comfortable towing? (I tow about 160 miles to the lake one way and it is not exactly flat driving.)

Thanks..........

The 20's on the on the 3.55 truck just effectively changed that 3.55 to a lower number. More like a 3.2 (or lower). Without knowing exact diameter of the tires/wheels the calculations are fuzzy.

The "aftermarket" 20's would have just made my decision against it. The 20's really messed with it's towing ability. A test drive of the two should be very enlightening.

 

[KM2]

Re: 3.55 vs 3.92

I'd go with the lower gearing for sure now that you mentioned 20's. I would guess the only time you would get better mpg with the 3.55 gears is unloaded highway driving.

The 4.7 isn't that big of engine for a quadcab truck plus 3500lbs.

 

[fishmen111]

Re: 3.55 vs 3.92

Go with the 3.92s. The difference is negligible. Your economy will not change much. The 20s have nothing to do with this. Only the rolling diameter of the tire is relevant when determining ratio. 20s will however cost you a little gas and power regardless of ratio. Moving the weight of metal vs. rubber out that far takes a suprising amount of energy to start moving. Dodges are gas hogs period. My '01 gets 10 on a good day.

 

[Silvertip]

Re: 3.55 vs 3.92

The diameter difference of the two tires (265-60x17 and 275-60x20) are dramatic at 3-inches (29.5 vs 33). Thats over a 10% difference so a 3.55 axle ratio is effectively down to 3.2 so the truck would very likely be laboring empty much less when towing. Go for the 3.92. The 20 inch wheels are the reason the other truck has 3.92 gears.

 

[redfury]

Re: 3.55 vs 3.92

If it has any bearing on the conversation at all, my Dakota has 3.55's with the V6. I can tow my 14ft Bluefin in OD all day long around here. However, when I have the 17ft glastron and the dual axle trailer it sits on behind it, I get no use of OD even if I wanted. I think though I'd be more interested in a larger V8 than worrying about the rear axle ratio in the long run. If you were using the truck for general hauling and runs to the hardware store, the 4.7 is okay, but I'd be more inclined to look for more power if I were hauling a heavy load 300 miles round trip ( that and making sure I have a tranny cooler ).

 

[Silvertip]

Re: 3.55 vs 3.92

A bigger engine is not always the best towing solution - especially if a poor choice in axle ratio is made. In the two Dodges in this post, putting 20 inch wheels and tires on a vehicle with 3.55 gears hurts performance enormously. The truck with the 3.92's is by far the better choice, especially with the 4.7 engine. Lets look at this by the numbers. Toyota for example advertises 401 lb/ft of torque on their biggest V8. With 3.2 gears (using the effective ratio from the big tires on the Dodge in this post), that translates to 1280 lb/feet of torque delivered to the ground, ignoring the torque multiplication effect of the torque converter. The 4.7 liter Dodge engine is advertised as having 329 lb/ft of torque. In the Dodge with the 3.92 gears, that translates to 1283 lb/ft of torque to the ground. So you can see the effect bad axle choice (or in this case, adding accessories without forethought) has on towing.

 

[chris4x4]

Re: 3.55 vs 3.92

FWIW, I had a Dodge quad cab 4x4, with the 4.7 and 3.55 gears and tow package. It had 32" tall tires on it and it was HORRIBLE to drive, empty or towing, it didnt seem like it wanted to pull very well. On grades it would go right to 3'rd gear ( out of 5 on the auto). I had 4:10 gears put in it and it became a different animal. VERY peppy, happy on the grades in OD, and my mpg. ent up 2 points, from 12 or so to 14.5 or so. If you get the truck with the 3:55 gears, change them to 4:10's. If you get the truck with the 3:92's, you should be fine.

 

[Silvertip]

Re: 3.55 vs 3.92

And if you buy a truck that has a tire swap, make sure the speedometer has been corrected.

 

[ebry710]

Re: 3.55 vs 3.92

I have the 3.92 rear end in my Dakota w/ factory 17" alloys, and wouldn't select that gearing again. Pig on gas, and high rpm for daily driver. Tows great though

I also had this configuration of Dakota and also the 3.5?. One was mine and one was a company truck. I had the 5.2 318. I though the 3.5(?) was great for speeding down the highway at 80 mph, but forget the overdrive when towing. My Dakota with the 3.92 could cruise at 65 to70 mph and didn't keeping switching gears all the time.

I think it is the balance between engine torque, transmission gearing and the differential gearing that makes the difference. The manufacturer really is the decider of that. For my 94 Dakota 318, 4 speed OD and 3.92 gearing allowed me to tow my 25' travel trailer. (braking is another thing). My company Dakota with all the same except rear end gearing couldn't do it. But that was a 1994 Dakota.

 

[Black Snow Slide]

Re: 3.55 vs 3.92

I had that exact truck a few years ago and when pulling a 5k pound trailer it had all it could do. I had the 4.7l with the 3.92s. After 3 or 4 times towing with that set up she went in and got 4.10s. Huge Improvement but gas mileage suffers. I also got a computer tune from a performance shop that drives it with a lap top plugged in. Another big improvement. You do those 2 things, and it will work very well for you. I would still have mine today but it was stollen.

 

[kyle f]

Re: 3.55 vs 3.92

I have had vehicles with everythign from a 2.73 ratio to a 4.10 and almost every stop in between. All I have to say is don't fear the gear. The RPMs and Fuel Economy is not that much different. The weight of the truck and aerodynamics effect it more than the gear ratio ever will.

I have never known anyone to wish they had went with a higher gear(lower numerically) an a lot of folks that wished they had went lower (higher numerically)