This would probably go better in a drivetrain only discussion however I would have put it in the 3rd gen area since I'm using one for the examples. I figured some of the guys with older trucks may have done something similar to this proposal and would have some input on it.

Hypothesis: A truck running a 4.10 rear end at say 55 mph will result in better mpg than the same truck running 3.73s at the same speed.

The basis of my proposal: Since a numerically higher gear ratio(I will be using 4.10 and 3.73's for my examples) results in an increased gearing advantage then shouldn't an engine turning that gearing be using less fuel to maintain a speed? Or to look at it a different way, shouldn't an engine turning 4.10's get better mileage than 3.73's with the engine turning the same rpms even with the same amount of wind resistance on the truck?

Another reason for my saying 55 mph will result in higher mpg for 4.10s vs. 3.73s is the fact that on a 3rd gen with an NV 5600 and stock (235/75/R17) tires will result in the engine turning at 1,600 rpm with the 3.73s (near lugging and hard to hold that speed) while it will still be turning 1,750 rpm with 4.10s

If compared at rpm to rpm, say 2,000 rpm where the 3.73s would allow the truck to run at 69 mph while the 4.10s would run the same truck at 63 mph the difference in air resistance will also allow the 4.10s to produce the better mpg numbers not counting the fact that the engine will turn at that speed easier.

Other Ideas: Unless the Nation's speed limits are reduced again to 55mph then the 3.73's are still going to beat the 4.10's on mileage for running 70mph (and higher) unless a heavy load is thrown into the mix. The 4.10s should result in better mileage numbers when towing heavy loads at 70mph, what the trade off weight is I am not sure, however when the engine is having to turn 2,500 rpm to maintain 78 mph (say running down I-10 through Fort Stockton, TX) then that may be a trade off point and the advantage move to 3.73s where it is only having to spin at 2250 rpm.

Does anyone have any proof/experience/opinions about this?

Despite it making it sound like I would like to switch to 4.10s I am actually wanting to switch to 3.42s so I can run lower rpm at higher speeds (1,850 at 70mph) in 6th gear and to be able to run 55-60 mph (2,200 at 60mph) more efficiently in 5th gear.

A better geared transmission would be one geared similar to a G-56 but with a 7th gear. Using 3.73s the G runs in 6th with a final ratio (.79) to the rear wheels of 2.95, I think a 7th gear of .65 resulting in a final ratio of 2.42 would give the best of both worlds, a better gearing advantage at low speeds and heavy towing with one shift away from being able to run higher speeds using less rpm.
Or one step further is an 8spd with gears of .80 (6th), .73 (7th), and .65 (8th) giving less rpm change between shifts and the option to find a more exact gear to match the speed and load of the truck.

 

Your hypothesis sound good but does not work in reality.

I had a 2003 3500, drw, 4x4, 6 speed with 4:10 when I was hotshotting, and I hated it all the times. I never had better mileage than 16mpg empty or 12mpg/11mpg loaded.
When I switched to a 3:73 truck, my mileage jumped 2mpg/3mpg more.

I alway try to keep trucks well maintained, and all stock, except for a small box (Edge EZ) to help mileage (every penny counts when working them).
Injectors were always kept under scrutiny, and I ran Amsoil synthetics everywhere.

The only advantage of a 4:10 geared truck is, in my opinion, short distance hauling. Where the speed is not a factor, or where climbs are frequent (hill or mountain country).

 

Running high RPMs on our cummins diesels just burns more fuel. I have 3.73 gears on my 06 with a G56. Fuel sucking hog, that best sums it up. Put on my 22.5 truck tires which lowered my RPM what a difference fuel mileage went up a solid 2 mpg.
Now a little over a month ago I put in a new G56 with the updated 6th gear .63 (I believe) Now at 65 I am running about 1600 RPM's Pulls just as good but mileage went up a 3/4 to 1 mpg
Pulling 38,000 at the speed limit and getting 11 mpg. Not to bad considering my truck sits 4 in higher then stock.

I have never seen a truck with 4.10's get better mileage then a truck with 3.73's

 

Valv, at what speeds and weights where you running?
And Ramrodd with the G 56 and 3.73s it is the same as running 4.10s on the NV 5600. I had heard there was a new final gear on the G-56 but I did not know what it was, if you still had stock tires you should be running at 1750 rpm at 70.

 

R..R..,
I see you are from the Dak's so you are well familar with snow.
I just pulled my GN racecar trailer loaded to 14k with a buddy's junk out of a parking lot with 12 inches of wet snow over frozen ruts. I only have rear drive but I use chains on the outside of the dualys. It churned up the snow and ice pretty good but I made it out without a struggle. I towed a few miles out to the interstate then took the chains off. It was near blizzard conditions and 4wd would have been nice but I managed very well as it was. I went 100 miles at 30 mph in 3rd gear same as everybody else on the unplowed freeway. The semis were going the same speed so it wasn't like I was holding them up.

I think 4wd alone would have been in trouble in the parking lot even as a dualy. I had spotted the trailer the night before in the previous snow so I really made a mess of the lot.

At the destination I put the chains back on and dragged the trailer up hill in a narrow alley again without problems even making the turn at the top of the hill into another lot where I spotted the trailer.

Today I'm up to my knees in wet snow but with the chains I get around.

As for mileage I get 20 at 60 mph on the highway. 18 in town. I get 13.5 towing heavy and 15.5 to 16 towing light. I have over 240k miles documented on mpg. There are times when it is not as good and times it is better so this is a good average done by spreadsheet. If I go 70 I get worse mpg so I just try not to run there. Aerodynamics enter big time after 65. You will pay for with with either gear ratio available. It takes so much torque to buck the air whether it come from gears or the motor. It certainly doesn't hurt the motor to buzz it just the wallet.

The big advantage of the 4.10 is the very low speed lugging. I can idle along at 3-5 mph in 1st with my trailers and go thru about anything loaded or not.

My truck is a southern truck and lots of the horse people use 2wd duallys for highway towing as they gets better mpg and less maintenance. When I got my truck I had planned on staying down south out of this snow emergency land. But here I am. I just pay a little more for AAA and try to not go where I will get stuck. So far the only thing I have used the AAA for is to haul my streetrod home after it quit on the highway one time.

 

^^^ Only 10-14 more inches of snow on the way

 

Bentwings sounds like your having all sorts of fun.

 

I'm learning about doing chains. Here are a few tips.
I use a 3 foot 2x6 cut at 45 on each end. I put this under the inside dually to install the chains. They just slip under the outside tire so install is easy. This only works on light snow or ice however. Other wise I lay out the chains evenly on both sides. I either back up on them or go forward which ever is easier. Go so you can wrap the chains with the connecting links so they will stay on top of the tires. Then hook the outside lever link so it just hangs in the first link. Hook up the inside link on the second or third link from the end. Then rehook the outside leverlink as tight as possible. Use lots of bungy cords to tighten the outside of the chains. Tighter the better. Make sure you get the extra links bound up on the outside. If they come loose they will cut the dually fender just like a can opener.

It takes about 15 minutes start to finish when it is snowing hard.

When you take them off just do exactly the reverse. Do not drop the inside connecting links as they will get stuck between the tires and can be a real nightmare getting out if you have a heavy trailer. I took me an hour to undo this mess when my 'buddy' says here I'll help and unhooked everything at once. I nearly had to take a tire off to get the chains out.

Also get some textured rubber gloves. Leather will quickly get soaking wet and very cold. I'm looking for some rubber sleeves similar to welding sleeves too. Nothing like being wet up to your elbows.

Have fun churning. haha

 

I think you should look at optimum engine rpm vs. gear ratio's if you are after the best mileage. My truck turns out its best mileage at 64 mph with 3.55's and a 6 speed manual trans. In my experience taller gears have always given better fuel economy vs low gears, low being anything over 4:1.
Just my .02

Kurt

 

Dually,

It's not as simple as gearing advantage or RPM.

First, two identical trucks with identical loads at the same speed on the same piece of road are doing the same amount of work. It takes a given amount of fuel to do a given amount of work. So, what are the variables? The engine is more efficient at certain RPMs than at others and it takes more power to spin an engine faster because of it's internal friction. If you run one truck with a lower (4.10) ratio at high speed to do the mileage test, the engine may be running at an inefficient RPM where it is having a hard time breathing and it is spinning it's internal parts much faster. Worse mileage will result. But slowing the engine down with higher gears (3.42) and lugging it up a grade, in OD, at full throttle and blowing black smoke with very high EGTs will not be efficient either.

Many guys simply equate RPM with mileage, but there is a lot more to it than that. The correct RPM for the load and the speed is what is important. Also, a tranny in OD is less efficient than one in direct. And wind resistance increases exponentially with speed. Larger tires reduce RPM but make the engine work harder, so before they can help with mileage, they have to more than makeup for a gearing problem. In other words, the engine has to be running very inefficiently because of high revs before you can get a net advantage by adding more drag to improve mileage. Not a good route for efficiency.

If you really want better mileage, get a more economical car, drive slower, carry lighter loads, run the engine at an efficient RPM when pulling, air up the tires, advance the timing, eliminate the EGR, reduce your number of trips, bla, bla, bla. All the things you've heard before. Simply lowering the RPM by going to higher gears is not the answer for every case. And neither is raising the RPM. It's doing the work at an efficient RPM and light pressure on the loud pedal that counts. Either way, a certain amount of work must be done to move the whole rig at a certain speed. Engine RPM is not always your friend or your enemy in that job.

You can test all this quite simply by running tests on the road or around town by either reving the engine way high all the time or driving a manual at very low RPM all the time, and see what results. Poor driveability and poor mileage. You won't get better mileage by red lining or by lugging.

When I'm on the open road and driving faster than I should be, I sometimes wish for higher gearing. When I have a load attached in the mountains, I sometimes wish for lower gearing. But overall, I think it just needs more gears, a higher final drive and a straight thru top gear with no OD. If I was really worried about it I'd throw a set of very skinny tires on and actually slow down, but sometimes, time is more expensive than fuel and skinny tires don't work as well as heavy knobbies.

 

Raspy, very good, that was something else that I was going to throw in there. When I was writing the first post I forgot to mention how I have been driving on the highway watching the boost gauge more than anything.

By not using the cruise control and keeping my boost below a certain level (13psi on the ISSPRO) when accelerating and going up hills (to which on hills I also go to that 13 psi before the foot of the hill so I have a little more speed advantage to get up the hill).
For normal driving I seem to get the best at 5-7 psi, I normally don't go over 10 psi (which is usually 70mph). I can tell sometimes if there is a head or tail wind by watching the boost gauge, there are times I can run 70 at 7psi if there is a tail wind.

 

Very informative thread. I posted in another section but will repeat it here. I am close to buying my first Dodge and first diesel (no good reason just want a diesel). It will be used mostly for touring and will sometimes tow either a small travel trailer (less than 6000lbs) or short trips with a boat that is less than 5000lbs. I plan to get cummins/automatic crew cab short bed 4wd. I am undecided on whether to get the 3.42 axle ratio that is now available for 2010 or the 3.73 that seems to be more standard and the ratio most dealers seem to stock. I think the high torque of the cummins would work well with the taller gear ratio (3.42) but I am unsure of the overall performance and which would be best. Dodge nor their dealers seem to have much info or advice on this. The advice given in the other forum seems to lean towards the 3.73 gears however it seems to me with such light tow loads and brute force torque the 3.42 might offer better overall performance. Any comments would be appreciated. thks, Ed

 

If you plan on doing any towing, even fairly light towing, and you are getting an '07 or earlier truck with the 48RE automatic, get the 3.73 gearing.

Yes the truck will pull the 3.42 gears but OD gets pretty tall with that ratio and bigger tires someday will really push it. Also, the first gear in the 48RE is not very low, so I'm always wishing for a different tranny when towing. I have the 3.73s.

Just because the Cummins has a lot of torque it doesn't mean you should use all it has pulling at low RPM. Better to keep it up to about 2000 or so when working it to keep the exhaust cooler and put less stress on the lockup clutch. If you do raise the power some day with a Smarty, the tranny will become the weak link if towing at low RPM.

I ran 35 inch tires for a while and OD was just out of the question in the hills when towing. That meant I really only had 3rd and second as my working gears when loaded. Finally, I wasted my torque converter lockup clutch by pulling grades at 1600 RPM and listening to the locomotive roar.

 

I plan to get a 2010 truck with the 6 sp automatic --- does your advice to go with 3.73 still apply? Thks for the advice, Ed

 

Ed,

I'd still go with the 3.73, but it's just my opinion. The new trannies are MUCH better and everyone seems to love them. Their top gear is, I think, the same ratio as the 48RE. So you'll end up with a lower low and more working gears, but the same on top. If you got the 3.42s you might pull a lot in 5th or 4th. I don't know. The only time the 3.42 makes sense to me is on flat ground unloaded. As it is with my gears I'm only turning 1500 RPM at 60.

After 185,000 miles and five sets of tires with four different sizes, I've concluded that the best all around tire for me is the Toyo 285/70/17. They seem to be the best compromise. They are an inch or so taller than the stock 265s and have raised the ratio a bit, so I'm already higher than the 3.73 would be with 265s. You might end up with this size too and they are nice. I'm also running a Smarty on the 90 HP setting and have been for about 150,000 miles. It really helps the Cummins feel more responsive.