Mileage with different transmissions.

[HCuervo]

If the tallest gear in the 5 speed automatic transmission is lower (Taller) than the ratio in a 6th speed manual: does that mean that the RPM's will be lower when cruising at 60 mph in the automatic trans and therefore the mileage will be better, all other things being equal (empty truck, stock tires, no trailer, etc. ). Does that discrepancy in ratios make the manual less fuel efficient?



I have a 2004. 5 3500 with the NV5400 and the mileage is well fixed around 13-14 mpg, empty (4. 01 rear). Do similar trucks with the automatic get better mileage?

Thanks

HC

 

[LDobie]

This should generate some good debate and thoughts. For the overall driving scheme of things, a Manual is more efficient. You are spinning the turbine and using a fluid as your method of energy transfer, your clutch provides a more positive mechanical lockup. From what I've seen on chassis dyno pulls with lighter duty trucks with the Hemi, the 2500's with the NV4500 were getting 15-20 more HP at the rear wheels, hence the more efficient drivetrain.



In theory, if you were to take 2 identical situations, the one that is spinning 200 or so less RPM should be getting better mileage, as there are fewer number of times that the injectors are spraying in fuel. In the real world, there probably isn't much difference between your NV5600 w/ 4. 10's and a 48RE with 3. 73's. Cruising at a light load, you might see a little better with the Auto, but you'd probably lose those gains under stop and go driving.



Lets hear some other thoughts...

 

[JVolpe]

Here are the ratios for the 48re and NV5600. I agree with LDobie, I have the 48re with 3. 73. With stock tc/trans vs manual I am sure that if you know how to drive the manual trans you should get better mileage in any city/urban driving situation. I am not even sure that empty cruising would be any better in the 48re, only if you never drop out of Lockup and then not much. All the mileage numbers that I have seen for the last two years seem to be a little higher for manual trans trucks.



48RE gear ratios are:

1st . 2. 45:1

2nd . 1. 45:1

3rd . 1. 00:1

4th . 0. 69:1

Rev . 2. 20:1



NV5600 GEAR RATIOS ARE:

FIRST 5. 63:1

SECOND 3. 38:1

THIRD 2. 04:1

FOURTH 1. 39:1

FIFTH 1. 00:1

SIXTH 0. 73:1

REVERSE 5. 63:1

 

[RSeverance]

I've got about 3500 miles on my NV5600 &3. 73 and average 15-16mpg. I have been trying progressive shifting, it's gotta help. I have a friend that has almost identical except short bed & auto. He is getting 19mpg! what gives-i swear I don't have my foot in it.

 

[Cattletrkr]

If the tallest gear in the 5 speed automatic transmission is lower (Taller) than the ratio in a 6th speed manual: does that mean that the RPM's will be lower when cruising at 60 mph in the automatic trans and therefore the mileage will be better, all other things being equal (empty truck, stock tires, no trailer, etc. ). Does that discrepancy in ratios make the manual less fuel efficient?



I have a 2004. 5 3500 with the NV5400 and the mileage is well fixed around 13-14 mpg, empty (4. 01 rear). Do similar trucks with the automatic get better mileage?

Thanks

HC

Where did you find a CTD with a 5spd auto?

 

[JVolpe]

Well I think he made an "Allisinian" slip, unless you count reverse.

 

[Hohn]

If the tallest gear in the 5 speed automatic transmission is lower (Taller) than the ratio in a 6th speed manual: does that mean that the RPM's will be lower when cruising at 60 mph in the automatic trans and therefore the mileage will be better, all other things being equal (empty truck, stock tires, no trailer, etc. ). Does that discrepancy in ratios make the manual less fuel efficient?



I have a 2004. 5 3500 with the NV5400 and the mileage is well fixed around 13-14 mpg, empty (4. 01 rear). Do similar trucks with the automatic get better mileage?

Thanks

HC

There are two things at work here that are in diametric opposition.



1) ON a diesel, lower rpm does not necessarily mean better mpg. On a gas engine is almost always does, because a gas engine has a relatively constant air:fuel ratio. Because a diesel varies the ratio of air to fuel in direct relation to engine load, it tends to offset any gains in mpg due to lower rpm.



So, the taller gearing of the automatic may NOT NECESSARILY lead to better mpg when cruising empty on the hwy.



2) Most people acknowledge that a manual transmission puts more power to the ground and loses less HP in itself. HOwever, when an auto trans is running with a locked up torque converter at very light loads, it is often MORE efficient than a manual trans is. This is simply because the planetary gear arrangement of the automatic is more efficient that the spur gears in the manual trans. That manual has all 6 gears constantly meshed, even if you are only using one. So you are driving 6 gears to only use one. The automatic trans' planetary arrangement is more efficient than this setup.



So, on paper the automatic appears capable of better hwy mpg because of its taller gearing. But we see that with a diesel, taller gearing isn't necessarily better. Better to be geared to run in the optimum BSFC range of the engine for max mpg.



We also see that the manual would appear to be more efficient because if its direct mechanical coupling. While this is mostly true, the advantage can all but disappear when the auto trans has a locked converter. In this situation it boils down to which automatic and which manual you are comparing.



It's pretty safe to say that as load increases, the manual trans will net an efficiency advantage due to the automatic trans not constantly run in lockup.



JM2CW

 

[HCuervo]

Gears and mileage

Thank you for the excellent discussion. I am retired USAF and did Gulf I in Doha and Incirlik.

HC