I'm thinking of putting more gear into my truck to help milage when towing. I tow about 11000 pound fithwhee right now. It pulls with no problem but I would like to know what it would do with lets say a 4:10 gears. I don't like to run real fast anyway so not to worried about looseing speed.

I will know in teh next couple of weeks. I added my PPE tuner about a month ago hoping it would improve my mileage. I got almost the exact same mileage on Tune 2 as I did stock but I did get a noticeable increase in power. Last week I installed 4.56 gears because I am running 37"toyo's. This brought me back to close to a stock ratio. It will be slightly over stock because my tires actually only measure out to be 35.5 inches in diameter. If you are running stovk tires and plan to install 4.10's you will probably be very close to my ratio. I will be filling my truck up for the first time this weekend since installing the gears and then I will be watching the miles. I would be happy to post hear when I get through the full tank with the results. Keep in mind the the majority of my driving to and from work which is about 12 miles round trip on surface roads with stop lights. Most of my long distance driving is in the winter when I leavfe my house to hit the trails on my dirtbike so it might be a while before I blow through a full tank of Diesel.

Mike

Mike

that would be great. Im hopeing it will allow the engine to work less resulting in higher mpg. I know that the cummins get only a decrease of 3 or 4 mpg running 4:10's.

Drivetrain.

You can simulate lower gears by simply holding a lower gear in the trans, our first gear is so short I can't imagine deeper gears doing anything to help mileage. If anything you would want 3.42's or 3.23's to keep the revs down when you aren't loaded.

A quick check would be to drop back a gear and keep the revs up.....no gear change needed.

I was hopeing it would put more power to the ground with less effort from the engine. Why is it that the duramax with all its power looses 50% mpg with a load.

You can simulate lower gears by simply holding a lower gear in the trans, our first gear is so short I can't imagine deeper gears doing anything to help mileage. If anything you would want 3.42's or 3.23's to keep the revs down when you aren't loaded.


Not really, having lower gears will allow you to take full advantage of ALL the speeds in your tranny, during accel. keep your rpm's lower and engine load lower as well.


A quick check would be to drop back a gear and keep the revs up.....no gear change needed.


I can't see why this wouldn't work to test mileage - IF you put on a tank worth of miles all on the highway. As soon as you start shifting, you won't be testing anything anymore.

I will know in teh next couple of weeks.


Keep us posted, I have been wondering.

I don't think the gear swap will net you much mpg, but that's my personal opinion.

Also, it may take a while to offset the cost of the materials / install. Just food for thought. I am however quite interested in this, keep us posted!

"keep your rpm's lower and engine load lower as well. "

This statement completely contridicts itself, there is no way to lower your rpm's AND the load on the engine with a gear change.....the only way is to lower the physical load against the motor.

You can lower the load by increasing the RPM's, that's what gears will do for a given speed. Increase your mechanical advantage by increasing the gear ratio with either rear end gears or dropping a gear in the tranny and the engine will have to make less torque to maintain speed....but it will have to turn more RPM's. There is no free lunch. The whole idea of maximizing efficiency is finding the sweet spot where the D-max makes the most HP (torque x rpm/5250) using the least amount of fuel. It seems GM found that 1600-1800 rpm is the sweet spot when unloaded, which is why that's the cruise rpm at 60 to 65 mph. What is the sweet spot when fully loaded? That is what we should be discussing........ Then we can adjust the gear ratio's to make sure we stay in that RPM range at a given speed.

A gear ratio is a gear ration, it doesn't matter if it's 6th gear with 4.11 gears or 5th gear with 3.73's.

Want to see how it would be with 4.11 gear just lock out the OD. You wont be happy with the results tho.

Now lets add some tech to this thread.....

Allison Gear ratio's:

1st 3.06
2nd 1.81
3rd 1.41
4th 1.00
5th .71
6th .61

To find your final drive ratio, you must multiply your rear end gear ratio by the tranny ratio.

Stock 3.73 gearing:
5th 2.65
6th 2.28

With 4.11's
5th 2.91
6th 2.51

So 5th gear with stock gearing is darn close to 4.11's in 6th, so basically it would be like giving up 6th gear and trading it for an even lower 1st gear. I don't know about you but with my 14,000 lb camper behind me I never need a lower 1st gear and I often run 6th gear on the highway.

"keep your rpm's lower and engine load lower as well. "

This statement completely contridicts itself, there is no way to lower your rpm's AND the load on the engine with a gear change.....the only way is to lower the physical load against the motor.

You can lower the load by increasing the RPM's, that's what gears will do for a given speed. Increase your mechanical advantage by increasing the gear ratio with either rear end gears or dropping a gear in the tranny and the engine will have to make less torque to maintain speed....but it will have to turn more RPM's. There is no free lunch. The whole idea of maximizing efficiency is finding the sweet spot where the D-max makes the most HP (torque x rpm/5250) using the least amount of fuel. It seems GM found that 1600-1800 rpm is the sweet spot when unloaded, which is why that's the cruise rpm at 60 to 65 mph. What is the sweet spot when fully loaded? That is what we should be discussing........ Then we can adjust the gear ratio's to make sure we stay in that RPM range at a given speed.

A gear ratio is a gear ration, it doesn't matter if it's 6th gear with 4.11 gears or 5th gear with 3.73's.


well said.

Now lets add some tech to this thread.....

Allison Gear ratio's:

1st 3.06
2nd 1.81
3rd 1.41
4th 1.00
5th .71
6th .61

To find your final drive ratio, you must multiply your rear end gear ratio by the tranny ratio.


So, if I read this correctly on a pre-2006 duramax with the 5 speed transmission, you would need to go to a 3.23 to get close to the 6 speed final drive.

Has anyone actually tried decreasing the final drive to get the RPM's lower for more efficiency when not towing or hauling (again, assuming the 5 speed with 3.73)? Are the additional rpms of the engine enough or a parasitic loss to actually have an effect on economy?

TLG

So, if I read this correctly on a pre-2006 duramax with the 5 speed transmission, you would need to go to a 3.23 to get close to the 6 speed final drive.

Has anyone actually tried decreasing the final drive to get the RPM's lower for more efficiency when not towing or hauling (again, assuming the 5 speed with 3.73)? Are the additional rpms of the engine enough or a parasitic loss to actually have an effect on economy?

TLG
And this would this put more stress and heat on the tranny when towing.

A quick test to answer your question would be for a 6-speed guy to lock out 6th gear and see what kind of mileage he was getting compared to running 6th gear. I bet you'll see about 1-2mpg unloaded in a highway situation. My commute is only 8 miles on back roads so I can't help you.

I'm not totally convinced that lower (numerically) gears would cause more heat. Heat in a tranny comes from a slipping converter (unlocked) and shifting. If anything you would be downshifting more with the taller gears which might cause more heat but I would venture to say that it's negligable.

And this would this put more stress and heat on the tranny when towing.

I don't believe that it would put any more stress on the tranny than the original "double overdrive" of the 6th gear. If your final drive ratio is the same, it shouldn't matter where it is stepped down. In fact, because of the reduction in the overdrive (.71 instead of .61), the amount of torque on the tranmission itself should be reduced by 16%.

Whe towing particularly heavy loads you should be in 4th gear anyway to minimize the torque stress on the transmission. In those cases, I would be willing to downshift since heavy hauling is typically less than 15% of my miles.

The idea behind slowing down the engine would be to minimize the parasitic losses of the running engine, particularly since there is so much readily available torque at lower rpms. Clearly that was the benefit sought with the addition of the 6th gear to the transmissions added in 2006.

Never been done? Is there a test that could be done, even with the DIC comparing the instantaneous fuel consumption on a 2006 in 5th gear and 6th gear?

I don't think the gear swap will net you much mpg, but that's my personal opinion.

Also, it may take a while to offset the cost of the materials / install. Just food for thought. I am however quite interested in this, keep us posted!


:exactly:

I agree completely. Regardless of what RPM or whatever, you are still fighting the wind resistance of pushing a 7000lb brick through the air at 75mph.

You want better fuel mpg? Leave your gearing alone, get a good towing/econ tune, and just slow down!!

If I go JUST highway unloaded cruise set on 71, I can get 20.5 mpg. And thats with stock gearing, 5 speed allison, and lower compression.

btw 3.42's is the lowest availible for our diffs

ben

I think in the latest Diesel Power Mag, that they did exactly this in an 05 LLY. I think they found a big of a mpg gain and they said it lowered egts as the engine wasn't lugging. Sorry, I don't have the exact issue around at work to let you know which one, but it is one of the latest issues from July or August.

Looked around for this article online...didn't see it. Got a chance to check for a link for those of us who don't get the mag?

I too am looking into a gear swap 'cause I pull a 1200lb toyhauler loaded across a 350 mile span and could use both better mileage and power. The only thing I've found so far is that when you increase your tire size you lower your rpm sweet spot. For instance I have 3.73 gears with 35 inch tires. My stock tires were roughly 30 inch (stock). My ideal rpm range lowered from 2600 to about 2000 @ 65mph due to a tire swap. According to the chart I found, I would need 4.56 gears to get my rpms back to the sweet spot. The equation is as follows [aftermarket tire size devided by stock tire size multiplied by stock gear equals new gear size]. Hope that helps. I still want to know if it's worth it or if i should just get a power programmer.

Honestly the easiest thing to do is to just manually shift the trans and drop a gear. If first gear isn't deep enough then you might look at swaping axle gears.

All you are gaining by changing axle gears is lower gearing down low and less gear up top.

If the next gear lower is too low, then swaping axle gears might help narrow the gap between gears....but with the allison that rarely happens.

I found it. Page 202 of August 2008 Diesel Power mag. They did the change to a 04 GMC 2500 running a 8" fabtech lift and 37x12.5x22 with Toyo MT's. Change was from 3.73 gears to 4.56

Results were:
- better acceleration,
- lower egt's
- shift points were better
- better mpg


So, draw your own conclusions but here is documentation where someone (running bigger wheels) did the change and did some tests to see what the differences are. You will still want a programmer, even if it is just to reprogram for the size of the tires to have an accurate speedo. But a little power is nice too, which gives back what the bigger tires took away.

That article just shows you that running the stock ratios is the way to go. They effectively just geared the truck to run the rpm it would have with stock tires and stock gears. That will for sure net better performance than a 37 inch tire with stock gears. That is to much gear.

I can't see where an increase rpm is going to net you better mileage. Every revolution of the engine results in fuel being consumed. The more rpms, the more fuel used.
Lower gears would give you more bottom end grunt, but I don't see where a Dmax needs more bottom end grunt. I agree with some previous posts, that staying in the sweet spot of the rpm band, you will maximize torque and mpg.
I don't think changing from 3.73 and 4.10 will net enough to offset the cost of doing that change. I would run it the way it is, or go with a larger tire, that would lower your rpm at a given speed, and could improve your mileage.

To give you an idea of what dropping rpm does for mileage, I had a 2001 8.1 with a 5 speed allison. and it got 10 mpg with a 4.10 gear. I had two dmaxes and now have a 2006 8.1 allison 6speed with 4.10 gears and I get 12.5 to 13 running the same roads. That is how much difference dropping those few hundred rpm did for mileage.

I found it. Page 202 of August 2008 Diesel Power mag. They did the change to a 04 GMC 2500 running a 8" fabtech lift and 37x12.5x22 with Toyo MT's. Change was from 3.73 gears to 4.56

Results were:
- better acceleration,
- lower egt's
- shift points were better
- better mpg

Post up the mpg #s when you get a chance please, no rush. That's what it seems most are concerned with here.

If you have stock size tires and go from 3.73 to a 4.10 you will loose highway MPG. Simple as that! Engine will be spinning more. I suppose you might have a gain in city if you could coach yourself to accelerate the same way as you used to but then again doubt any of us has that much patience.

I found that best mileage and actually getting there is 62MPH. It seems to off set windy days to a degree also. If you go better the 65 towing then don't even bother talking about towing MPG. Aerodynamic HP from that trailer is eating your lunch!

quote=Duromax04;2832051]
I can't see where an increase rpm is going to net you better mileage. Every revolution of the engine results in fuel being consumed. The more rpms, the more fuel used. [quote]

So during a "load" why are EGT's lowered when downshifting/inmcreasing RMP's? RPM is only one contributor but load of the engine - how hard your engine is working adversely impacts the MPG more than RPM.

I too have 35's (LLY) and looking at either 411 or 456 for faster accell & lower egt's. If I improve MPG it's a bonus.

Also, bigger tires = more rolling resistance and more aero drag (especiually if lifted) though a programmer (and light foot) will offset it - that's been my experience.

Read the Ford diesel forums before you do this. Ford offered 3.73, 4.1 and 4.56 gears in 6.4 diesel trucks. (from memory) The 3.73 trucks get 14-16 mpg on the highway empty, 10 loaded. The 4.10 trucks get 12 empty, 8-10 loaded and the 4.56 trucks are under 10 all the time.

There may be a few occasions were steeper gears will give better mpg's, but that is normally only when towing heavy with much taller tires than stock. 99% of the time lower rpm's mean lower fuel consumption and better mpg's.

Still, it's your truck. Steeper gears are always more fun to accelerate with! Enjoy

jb

Post up the mpg #s when you get a chance please, no rush. That's what it seems most are concerned with here.


thats all the article said. It wasn't my truck.

"how hard your engine is working adversely impacts the MPG more than RPM. "

So the question is: Will an increase in rpm offset or more the added force to move the load with a taller gear. I think the lower gear will still average out less mpg, because at cruising speed, it will have to fire more times every minute and that will use more fuel over the long haul, unless maybe you do all town deliveries, and maintain a slower speed.
Part of the function of engine load is the actual weight it is pulling, gear ratio, and most importantly how hard your right foot is planted on the pedal. If you lay into a truck heavy, you will put more load on it, then if you accel gently. That would use more fuel no matter what load or gear you have.

And, if you have to much gear, or to large of tires, you can go to far that way as well. However, there is a middle ground, that will yield good accel and the most optimum mpg. I would like to believe that GM got pretty close with 3.73's and 245/ or 265 tires. Going lower gears is going to cause the engine to run more rpm at any given speed because it is a fixed ratio. So, unless it is insanely over geared, the taller gear should net you the better mpg, within reason.

You are correct that larger tires drag harder, grab more air, and generally are heavier than smaller ones, so any gain you would get in gearing with them, would probably be lost in parasitic drag. Takes more power to get a heavier tire rolling than a smaller one.

This might be a good time to ask here? There never seems to be an charts on the Duramax with BSFC and VE? Brake Specific Fuel consumption and Volumemetric efficiency?

thats all the article said. It wasn't my truck.

Yeah, wasn't crackin' on ya but...when it just says "improved mpg" I tend to look for more info. Thanks for posting up the pic.

Alot of those magazines will test drive a brand new truck (for example) and then say "we went X miles and topped it off with Y gallons, yielding Z mpg." Typically it's some really short amount of distance, I saw an article where they rated a Chevy at 10mpg or something over 50 miles. That's not really a fair way to report figures like that.

Let Me Simplify Things For All Ya.. Engines Get The Best Efficiency At Peak Tq.. Accordingly If Your Gear Your Motor To Run At Peak Tq You Will Bt The Best Mpg...

Diesels Have Peak Tq Way Early In The Power Band So Accordingly You Dont Ned Much Gear To Take Advantage Of This

Also Form What I've Seen Most Diesels Have A Relatively Flat Tq Band So That Would Mean You Wont Get Much Change Either Way You Go On Gearing

Id Bet To Say Steeper Gears Can Help Trucks With Big Tires B\c They Are Running Too Low Of A Rpm...

A Simpler Solution For Mpg Purposes Is To Just Raise Your Pt Shift Points.. I Did This And It Seemed To Help ( A Little)

Simply adding the second overdrive (6th gear) for around 1600 bucks is a nice option for MPG improvement empty and when towing just slip down a gear if too high.

after reading through this entire thread, I'm still uncertain. If I am running 35" toyos with the 6sp allison. Is it worth my while to upgrade to 4.56s? Or do I just go with a programmer? Best mpg at 65mph is about 14 right now.

I think there may be a discussion of a couple of different issues going on. What may get better mileage empty cruising on the highway may not necessarily be what gets better mileage towing 12000 lbs. Larger tires just add another dimension to the whole issue. It is always good to get the results of real world testing or experience, but you may have to be careful about drawing conclusions from the tests if the setup was different than yours.