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I was looking at a calculator and it said that with 4.56's and 35's, I'll be at 3000 rpm at 70. Is that okay for the motor?
 

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I wouldn't want to be running at 3k trying to keep up with traffic on a long trip. Why are you considering such low gearing?
 

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That doesnt sound too good to me.
 

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a lot of guys tow with the stock 3.73's and 33 or 35 inch tires and do fine... i would like my motor running as little rpm as possible (within reason of course)
 

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That's to low of a gear for 35's if you really want to change your ratio I would consider 4.10's. Like others say you don't need to for 33 or 35's. I towed for a while with the stock gears on both 38's and 40's and towed fine I now have 4.56's.
 

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I got the idea for 4.56's from this forum! Some are saying yes and some are saying no...I don't get it!
 

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I wouldn't go that high. 4.10's IMO.
 

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i think the guys that are saying yes are the ones like me that are running 37" or larger tires. i pulled plenty when i had 35" and the stock gears and it didn't do anything to the truck felt the same as when i had 285 almost i think you would be waisting your money if you put the 4.56 gears in and only ran 35" if you really want to put gears in i would go with a 4.10 but then you run into the problem alot of us that run different gears are haveing and you will get a service brakes soon light come on and then you won't have ABS so if you are just running 35" tires stick with the stock gears you will be fine
 

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The one thing no one mention is fuel economy! WOW!! not to mentionall the rest of the wear and tear!!:eek::
 

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did anyone experience beeter mileage town or highway with 37's or bigger when changing gears to 4.56's i wanna go 40's but i still have stock gearing
 

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i went from 35" to 37" and changed my gears i keep getting about 15 mpg
 

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It is my understanding that 1800 RPM is the optimum RPM for the Duramax to get the best combination of torque and milege. Use your calculator and shoot for that RPM at the speed you want to drive.
 

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With bigger tires, more gear will help everything!

OK, I run 35's and I am also running 4.56's. I also have the 6 speed trans. I think these trucks should have came with a bit more gearing from the factory 3.73 with the new trans. Anyway, everyone wants to say how lowering the rpm helps gas mileage, well that only works to a small degree. When I installed the 35's my rpm's fell a bunch, at 75 mph I was only turning 1700 rpm. That should equal great gas mileage according to most people on here. Well let me tell you, it doesn't. Try about 10 - 11 mpg at freeway speeds in 6th gear. ON a 3000 mile trip I experimented and pulled it back in 5th gear which brought the rpm's up to 2000 at 75 and picked up to 12 - 13 mpg. How can that be? More RPM and better mileage, exactly what most say can't happen. I took a chance (because no one recommended 4.56 with 35's) and went with MY gut feelings and boy was it the right decision. Most of my miles are not towing, lots of freeway miles and it was the perfect choice. I now get 14 - 15 on the highway, the EGT's are cooler, the trans shifts better (and its a lot easier on the trans) and the power from low or high speeds is unbelievable! Sorry for the long post, but most people giving advice have heard, or have an opinion, but haven't done it. I have and it's perfect if you have the 6 speed.
 

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dockrocker;1596626; said:
OK, I run 35's and I am also running 4.56's. I also have the 6 speed trans. I think these trucks should have came with a bit more gearing from the factory 3.73 with the new trans. Anyway, everyone wants to say how lowering the rpm helps gas mileage, well that only works to a small degree. When I installed the 35's my rpm's fell a bunch, at 75 mph I was only turning 1700 rpm. That should equal great gas mileage according to most people on here. Well let me tell you, it doesn't. Try about 10 - 11 mpg at freeway speeds in 6th gear. ON a 3000 mile trip I experimented and pulled it back in 5th gear which brought the rpm's up to 2000 at 75 and picked up to 12 - 13 mpg. How can that be? More RPM and better mileage, exactly what most say can't happen. I took a chance (because no one recommended 4.56 with 35's) and went with MY gut feelings and boy was it the right decision. Most of my miles are not towing, lots of freeway miles and it was the perfect choice. I now get 14 - 15 on the highway, the EGT's are cooler, the trans shifts better (and its a lot easier on the trans) and the power from low or high speeds is unbelievable! Sorry for the long post, but most people giving advice have heard, or have an opinion, but haven't done it. I have and it's perfect if you have the 6 speed.

When you changed to 35s you did more than lower the cruising rpm of your truck. You also significantly increased your ground clearance and the cross-sectional area of your truck with the taller and wider tires. That means much more wind resistance. You also added more weight (rotational weight at that, which has greater impact).

Fuel economy can be largely controlled by the brake specific fuel consumption (bsfc) of the engine. The bsfc is a uniqe three dimensional plot for every engine of load vs. rpm vs. fuel consumption (in volume per time-i.e. gallons per hour). Places on the plot with lower fuel consumption per horsepower generated are areas with greater efficiency of the engine.

When you put the 35s on you increased your wind resistance which means more load on the engine, and at lower rpm (because of the taller tires). Your decreased mileage in 6th with the stock 3.73 gears means that you were operating below the peak efficiency of the LBZ at that given load (75 mph). When you went to fifth gear this put you closer to that peak efficiency which equals better fuel mileage.

Switching to the 4.56 and using sixth is effectively similar to 5th gear and the 3.73 when cruising along steady state at 75mph.

I think people who would benefit from lower rpm are the 5 speed allisons with stock tires who already do 2150ish rpm at 75mph.
 

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The question referred to a lifted truck with 35's. I see now he probably has the 5 speed. A 4.30 gear would put the effective gear back where he was, a 4.56 might be a little steep for a five speed but he will only be turning 150 RPM more than he was with the 3.73 and the factory pizza cutters. I do have a very good friend that has an 03 with 4.10 and the same 35's I run and it could use a touch more gear. The only way I would ever suggest staying with the 3.73 is if the cost is in the way, but you WILL save it over the long haul.
People get carried away with this <less> gear = fuel mileage thing. If you had one of these power trains in a 2wd Colorado you could probably get away with that theory. But in the real world most of us are crew cab, 4x4 and these things are like a heavy brick BEFORE we make it worse by adding a lift and larger tires. The 3.73 gear ratio was picked by GM because thats as high as you can get away with under perfect (and stock) conditions, increase the gear just a touch and I promise you it will be detrimental to fuel economy. Even with stock tires on a long grade or against a head wind, pull it back into fifth at the same throttle position and watch your speed increase.
 

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dockrocker;1596945; said:
The question referred to a lifted truck with 35's. I see now he probably has the 5 speed. A 4.30 gear would put the effective gear back where he was, a 4.56 might be a little steep for a five speed but he will only be turning 150 RPM more than he was with the 3.73 and the factory pizza cutters. I do have a very good friend that has an 03 with 4.10 and the same 35's I run and it could use a touch more gear. The only way I would ever suggest staying with the 3.73 is if the cost is in the way, but you WILL save it over the long haul.
People get carried away with this <less> gear = fuel mileage thing. If you had one of these power trains in a 2wd Colorado you could probably get away with that theory. But in the real world most of us are crew cab, 4x4 and these things are like a heavy brick BEFORE we make it worse by adding a lift and larger tires. The 3.73 gear ratio was picked by GM because thats as high as you can get away with under perfect (and stock) conditions, increase the gear just a touch and I promise you it will be detrimental to fuel economy. Even with stock tires on a long grade or against a head wind, pull it back into fifth at the same throttle position and watch your speed increase.

I concur. This man speaks the truth. Less gear will not increase your fuel mileage in these trucks. Fuel mileage suffers as soon as an engine starts to lug, even a little bit. And if you're turning 1700 RPM at 70 MPH, even the almighty Duramax is lugging, and your MPG will be worse that if you're spinning 2100 RPM at the same speed (less luggging). My real world experience involves switching to a 235-85-16 tire. A tire that's skinnier than stock (reduced rolling resistence), yet taller (less effective gear) should be good for an MPG increase wouldn't ya think? Nope. Every tank hand calculated, corrected for increased diameter = loss of just over 1 MPG average over 30K. Now, there are other factors such as different tread design and raising the height of the truck by about 0.63", but I think most of the MPG loss is due to the change in effective gearing. If you can afford to, I would certainly regear to get back to the factory effective gear ratio, maybe even a bit more gear if you're towing much at all.
 
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