i agree i had 305-70-17s on my truck and it killed my mileage. i dropped to 285's and can get around 20 on the highway with 3.73 gears and 6 spd. i could get 22 with the truck stock.

 

Sounds about right! My built DTT cruised the interstate at 130-140* during the summer.... in clear air. Stock trannies have a lot more slippage and will run somewhat higher.
Anywhere under 170* (sustained) is acceptable and healthy...IMO.

RJ

 

I have 3.54's with 285's and an automatic. I don't really like it much and was a lot happier with the 3.54's and 265's I used to have. I do like the 285's better than the 265's so I'm considering re-gearing to 3.73's.

 

Good one.... needed that laugh...
And thanks for your service.

 

The following is objective and will apply differently to each individual truck.

I like what people are saying in this thread. I have 35's with me 3.55's and i still hand calculate about 22 on the freeway. That's in the slow lane, flat for 350 miles, with the wind at 55mph. Towing has been hard on my truck though. I made two trips with about 8500lbs, for 800 miles, and burned up my tranny after the second trip. The research I have done has shown, 3.55 with stock tires (265) is the best scenario (gear ratio combination) for the longevity of your trucks life. If you raise the tire size, it raises your gear ration even more, and changes the way your truck handles with and without a load, good and bad. My 3.55 with 35's is somewhere around 3.35 I think. If you look around there are links to calculate gear ratio to tires size, and tire size to gas mileage. Work the number and see what works best for you. I would look at 3.73 with 285, but you will have to do the math and see what works best for you.

Or instead of changing your gears, do the math and maybe raise the tire size of your current tires to raise your gear ratio. I dont know if you could raise it enough, but 35's might take 4.10 to 3.80 or 3.90 and increase your gas mileage.

 

I'm running stock 2x4WD with 3.55 (which makes a difference on the MPG no 4x4) with 285 at 65 psi towing 3 horse trailer full ( 10.5k) I run at 13.8 MPG @ 65MPH ( my F350 ran at 7.9 to 8.5 MPG), no load i'm in the 23 MPG plus ( with my slippers on, and the lead boots on the seat next to me).

 

This is not directed directly at daddy mopar, but to all of you that have other than stock tires and are stating hand calculated milage number. When you changed the tire size from stock, did you have the Rev/mile number reflashed in your truck ECM? If you did not, then your milage numbers are bunk since the different tire size will throw off the speedometer and the odometer. Now if you DID reflash the computer, then the numbers are what they are.

 

I know my speedo is off about 5mph. There was almost a 50 mile difference between the garmin and what mileage the truck odometer read.

 

Mechanos, no harm done. I have a nifty GPS that keeps track of speed and miles traveled.
However, I too am thinking of upgrading my 3.54 to 3.73. My research has shown the 3.54 and the 3.73 carriers are the same, making the transition easy with the purchase of a ring and pinion. PM with your findings, I am interested in what you find, relating to price and reliability.

 

How hard is it to change the ring and pinion gears? And how much do they cost? At 2K RPM I'm going about 55-60mph with OD locked out, 80mph with it on. I'm going to be pulling a 15K fiver next year and fear my 315/70R17s with 3.54s won't cut it. I might just go back to 285/70R17s....

 

Its not to hard to change the ring and pinion, just time consuming. I have heard that most shops charge about 800-1000. I priced out the 3.54 gears. It was $1000 for everything using top of the line stuff, but i don't think you need to buy a new carrier going from 3.54 to 4.10. so that is about $200 less. My truck ran 2k at 65mph with 285's. I gained about 1.5mpg going from 4.10 to 3.54, and when I need the pulling power at lower speeds I just knock it out of OD. Having had both ratios and if was going to go through the hassle of changing the gears, the 3.73 ratio would be the way to go. IMO

 

A R&P swap isn't that technically hard, but it must be done precisely. You want someone doing the work who's had experience setting backlash and bearing preload.

Your rig will struggle with your 5er trailer. This is because the factory TC is so loose that there is a radical difference between locked TC clutch and unlocked. Your gearing is too tall to run locked. Run unlocked in 3rd and you'll heat the trans tons and burn up all your fuel at 2400rpm or so!

Towing with heavy with a stock Dodge automatic limits you. You need to either stick with stock size tires or re-gear with bigger rubber.

With a set of 4.30s or so, your truck will be a towing machine that will perform well with 315s. 4.30s with 285 tires would be a towing monster. You want your 2K rpm range to fall right about 62-65mph (in OD) for towing. This gives you enough lattitude to slow down (to 55-60) and still have good power, but also lets you speed up to 70ish or faster without screaming RPM-- all with the OD engaged and TC locked. I don't even consider speeds past 70 for towing calculations. Anyone who tows a big trailer with a pickup going faster than 70mph gets my scorn for bad judgment. (JMO).

Gear options very by manual trans (D80) and auto (D70), so you might not have the same options someone else has.

JH

 

One other comment on A/T mpg when re-gearing:

Auto trannies slip as a function of load. When you go to taller gears, it increases the torque load on the converter and increases slippage.

So, if you REALLY want better towing MPG with an automatic, gears are not the place to look-- the transmission is. A quality aftermarket converter and valvebody kit will reduce slippage, increase efficiency, lower temps and improve MPG when under load.

For towing, you want tighter trans and shorter gears. Some in this thread seem to be proposing taller gears with the factory (loose) trans for towing-- a step in the wrong direction, imo.

If I had a dedicated A/T pickup that I used primarily for towing, I'd go straight to 4.56s with 285 tires and never look back.

With the manual trans, you can get away with more taller gearing (and need it with the shorter OD ratio of the manuals vs the auto (.71/.72 for manuals vs .69 for auto).

We all want better mpg towing, but a little more RPM and less load is a good thing. Higher RPM gives you more fluid circulation and places less load on the engine. So gear shorter and slow down, I say

 

Well said HOHN! I posted about tire ratio to gear combination, but HOHN makes a good point. Gears would be an easy way, tranny would be a correct way.