Diesel gear question

[gwvwadc]

Still looking at upgrading to diesel truck to tow with.
As I look around, I see rear differential ratios from 3.55, 3.73 and up. Tow ratings don't seem to change from one to the other for 3/4 ton truck.
Full load for truck will be MAXIMUM 10,000 lbs. (that is TT, us and stuff).
Recommendations? (Again as in earlier post - not interested in particular brand, just tech information).
Thanks,
(tx)

[Jim W]

Are you looking for a new truck say from 2013 and up? If so you may only be able to purchase a SRW diesel truck with only one gear set no options are offered. It seems that all the big three are only offering one gear ratio for a SRW diesel engine trucks. You can thank the government CAFE rules. The DRW trucks can be offered with the 4:10, 3:73 and 3:42 differential gearing.

Each manufacture is offering a different ratio than the other as example the Ram trucks offer 3:42 differential with either the 6spd 68RFE auto or the 6spd AISIN auto. This is regardless if you buy a 3500HD or a 2500HD truck with the Ram trucks with a diesel engine.

Jim W.

[JRTJH]

It appears that with the diesel option, GM (GMC and Chevy) only offer a 3.73 differential, but Ford offers two options, 3.31 and 3.55. Not sure about Dodge,

Depending on the manufacturer you choose, there are some differential options available in some lines, but not from all of the manufacturers. Go to Chevrolet.com, Ramtrucks.com and Ford. com. Just do a "build my own truck" to run through the different options available. Also, hit the "specs" pages on those websites to see the differences in towing capability/payload based on different options. As an example, Ford offers the F250 with a 9900 GVW and a 10000 GVW. There's only 100 lbs difference between the two, but depending on the state you live in, there could be several hundred dollars a year difference in the registration fees.

Do your homework !!!!! then go shopping.

[abneynormal]

the lower the ratio the higher rpm for the same speed unless you can offset it with a transmission option. I don't have the problem with towing my th since I opted for a truck truck to pull it with. the rear end is a 3:73 with a 9 speed overdrive eaton/ fuller. in od at 70 mph im turning 1600 rpm. the more rpm you turn the more fuel you burn.

[rnkburg]

Towing I have used 3.55 3.73 and 4.11. I like 3.73 the best , 4.11 are great in town but the 3.73 seems to be the best all around and not wound out at 65



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[jerseyphil]

My 2011 F250 SRW DIESEL has the 3.31 rear. It easily pulls my 2011 Cougar 318SAB, which has a fully loaded weight of about 12,000.

[Wayne]

I switched to diesel early this yr. I found an almost new 2011 Ram (our 1st Dodge) that had everything we wanted. Ours came with the 342 (411 is also avail). Our 5er generally runs 9000-9500 lbs loaded. I switched from an 08 GMC 6.0 w/373. While it was a great vehicle, there is no comparison for towing between gas & diesel. I can use cruise almost all the time & that makes the long trips so much easier. Get the Ford, GM or Ram that you like & enjoy.

[Avofarmer66]

Torque is pulling ability and HP is speed.

The Ram Cummins 6.7 has 3 torque offers. 600 lb. ft. is the standard for the 2500, 800 lb. ft. is the option (called High Output) the 3500 offers the 800 and the 860 lb. ft. torque. All are 6 cylinder. Most of those torque offering started in 2011.

Cummins sells this engine to Freightliner and many different commercial applications, generators, dirt equipment, logging....

The Ford offers 800 lb.ft. I not sure about the DuraMax 650???

My 2012 2500 Ram has the 6.7 High Output and we pull a Montana 3400RL loaded most times we're at 14000 lb. +- 200 lb. (grand kids) Pulling I80 Downer Pass into Nevada we did that in cruise at 65 mph average of 9.5 MPG on the lie-O-meter, from Lake of the Springs TT to Bridgeport CA

if you had a gas motor the torque for the 6.2 Ford is 450 lb. ft. you need the 4:10 gear ratio.

[pbryan]

Check out this calculator:
http://www.crawlpedia.com/rpm_gear_calculator.htm
You can see the actual difference in engine RPM as the axle ratio changes.

That is really the only thing the axle ratio does for you... Take the Ford for example, the 3.31 will give you lower engine rpm at a given vehicle speed, assuming the same wheels, tires, and transmission. The 3.55 gives a higher engine speed.

The higher engine speed gets you in to the high torque and power regions of the engine's capability sooner as you accelerate. Thus you can get better acceleration performance from a higher gear ratio. This is why many people think they are best for towing.

The trade off is that when you are cruising on the highway at light loads, you will be running at a higher engine rpm, even though the power isn't needed. This is generally worse for fuel economy.

It's up to you if you prefer the higher performance potential or the better fuel economy potential. I can tell you with the Ford, you won't have any trouble pulling 10k with the 3.31 on the 6.7L, but the 3.55 might get you a little faster 0-60 time.

In some cases the wheel/tire choices dictate the axle ratio too... i.e. in Ford's you can't get 20" wheels with the 3.31, but if you do the math the 20" wheel/tire combo with the 3.55 is almost the same engine rpm as the 18" wheel/tire with the 3.31...

--Paul

[JRTJH]

Another fact to consider is that with the new 6 speed and 8 speed transmissions that lock up in all gears, you can select a lower gear to maintain the desired engine RPM range. Take for example, you have a 4.11 rear end and a 6 speed transmission, you can "cruise" in 6th gear at 2800 RPM. That's "just below" the peak torque for most gas engines.

As a comparative example (not exact numbers) take a 3.73 rear end, the same engine, same transmission, you can "cruise" in 5th gear at 2800 RPM. Same "just below" the peak torque, and when you're "solo" (not towing) you have the option of upshifting to 6th gear and cruise at a lower RPM than is possible with the 4.11 rear end. With most 6 speed transmissions you'll find a "super low" first gear, two intermediate gears, 4th is "normal drive" and then 5th and 6th are "overdrives". As long as the torque converter locks up, you'll get the same benefit in fuel economy and reduced heat generated whether you're in 4th, 5th or 6th gear, so it's relatively easy to operate where the engine RPM reflects peak torque/HP production.

Granted, those two "lines" (torque and HP) cross at a lower RPM in diesel engines, and the best operating RPM should be lower, but the same can be achieved in gas engines. Many people just "put her in drive and go" and they will usually be the ones complaining about "the cruise keeps downshifting" or "I can't use cruise because it wont anticipate the hills" or a number of different complaints. Driving while towing a large load is seldom an "set it and forget it" kind of task. There's always the need to stay vigilant to what's ahead, anticipate speed/hill/braking requirements and react before the truck goes "UGG" :)