Gearing

[RHuot]

This has probably been discussed many times but this is the first time in here for me, please be patient.

I am in the middle of spec out a new 3500 Mega Cab single wheel. I have been reading the TDR magazine and following Cummins in the Dodge for quite some time. I am very excited about owning the package that I have dreamed of. This truck will have an additional 70 gallon transfer flow tank and a splitter of some type (yet to be determined). My goal is maximim fuel mileage. Down shifting is not a problem. So this promted some questions for the seasoned veterans.



My bottom line is which gear should be used for the best fuel mileage? I understand the first gen 5. 9 are capable of 30mpg. Was it because of the 3:55 gear ratio? What if the new 6. 7 had the 3:55 gear? Is 30mpg attainable? It appears to me that the 4:10 is only there for factory bragging rights and has on real positive applications (aside from stump pulling). The 3:73 is better but still not optimum, even with a gear splitter it still would fall short.

So the questions would be. . Can you get a 3:55 gear in the AAM 11. 50 housing? Should this be investigated? Has anyone looked into this? All comments are welcome.

Thank you



Snow Pro

 

[Steve Graham]

30 mpg?

When my 92 was new and stock, best mpg was 23 @ 65mph. I don't know where you heard or read that 1st gens can get 30 mpg, but I don't think it's reasonable. Now my Scout with the 4B does occasionally get 30 mpg but normal is 25 mpg.

 

[mberry]

My bottom line is which gear should be used for the best fuel mileage? I understand the first gen 5. 9 are capable of 30mpg. Was it because of the 3:55 gear ratio? What if the new 6. 7 had the 3:55 gear? Is 30mpg attainable? It appears to me that the 4:10 is only there for factory bragging rights and has on real positive applications (aside from stump pulling). The 3:73 is better but still not optimum, even with a gear splitter it still would fall short.

So the questions would be. . Can you get a 3:55 gear in the AAM 11. 50 housing? Should this be investigated? Has anyone looked into this? All comments are welcome.

Thank you



Snow Pro

Your best combination for fuel mileage is going to be the manual transmission with the 3. 73 gears. If you do most of your driving on the highway, the best thing for mileage is keeping it under 2K RPM. You cannot get anything but the 3. 73 or 4. 10 currently. I sure wish there were other options available. AAM does make other gearing options, but apparently not for the Dodge application and it has been investigated.

 

[EKlem]

Your engine has an optimum rpm that is load dependant. At full load(wide open throttle), your engine will be most efficient somewhere right around torque peak. At no load, your engine is most efficient at idle. Everything else falls somewhere in between. With 245/75/16's and a 3. 55 rear, I turn about 2100rpm at 70mph. Obviously the engine is spinning faster than optimal but it also has more power at that rpm. If you are going for mileage, I would suggest getting a pyro. If the pyro is climbing quickly and your engine is not putting out a lot of power, your rpms are too low. Lugging is just as bad as running high revs.



The other thing to think about is overall speed. The air resistance equation has several terms but by far the dominant one at highway speed is the squared one so you get a lot more resistance for a little more speed.

 

[RHuot]

Thank you for the comments! So based on the discussion the six speed with the 3:54 gear would be optimum. What about adding a overdrive gear splitter (USGear dualrange)?

2100 @ 70mph is getting close, I was thiniking around 1800 @ 75 is the goal Any thoughts???

I aggree that the higest efficency is within the torque band. I previsously had a 2000 Excursion 5. 4, V8, 4:10 gas and I was able to get a best of 14mpg and an average of 12 city/highway great for that unit. I would not use the OD unitl I was over 3,000 rpm because that engine had a high rpm torque band. I 'm looking forward to the completely different world of low end torque but the foot rules fuel milage. I currently have an email into AAM with questions regarding the 1150 axle and the 3:54/5:31 gear spred they state in the information section. Some of you have probably already been there.

 

[dpuckett]

My suggestion based on company/ fleet trucks I've owned in the past is to work the engine from day one. Not abuse, just work. Let it get warm first. DONT BABY it, or it'll never break in.



Beyond that, learn to drive by the pyrometer, like EKlem said. Takes fuel to make heat, so keep the heat vs speed to a reasonable ratio. Sometimes I can drop 200deg by dropping 4-5mph. Unless I am going cross country, it is a negligible time difference. Other times, I can go 4-8mph faster and only gain 50deg, thereby actually gaining MPGs in the long run.



Daniel

 

[MLisle]

There was a article in Diesel Power magazine about a Ford Powerstroke that made 30mpg. It was a 2wd that was lowered with spoilers and the rear diff had 3. 09 gears, so its possible if you want to spend the money. Swap in a dana 80 with the lowest gears possible, lower the truck and add spoilers. You might get bragging rights for lowest fuel consumption/max range on a tank of fuel but not anything else.

 

[p-Bar]

There was a article in Diesel Power magazine about a Ford Powerstroke that made 30mpg. It was a 2wd that was lowered with spoilers and the rear diff had 3. 09 gears, so its possible if you want to spend the money. Swap in a dana 80 with the lowest gears possible, lower the truck and add spoilers. You might get bragging rights for lowest fuel consumption/max range on a tank of fuel but not anything else.

with a tail wind. I'm sure the mileage sucked around town.