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Discussion Starter #1
I currently own a '95 Cummins Dodge 4x4 with an auto trans, 4.10 gears, and 285/75 16 tires. It is mostly stock right now with an intake and exhaust being the only real modifications that have been done to it so far. Set up the way it is right now, the truck will pull ~19 mpg on the highway if I cruise at 55-60 mph, but if I go up to 65+ the mileage drops off considerably. With fuel prices the way they are it wouldn't take long to cost justify regearing this truck, but I'm trying to decide if it is worth it or not. Do you guys think it would be reasonable to assume that I could get ~19 mpg at 65 mph with a set of 3.55 gears in the axles or do you think that wind resistance is going to take a bigger toll on my mileage than spinning the motor faster will?

FWIW, my mileage figures are with the truck unloaded or with only a pickup bed load in it. I don't really care as much about what my mileage is when towing as I don't tow often enough for it to matter much.

Thanks in advance!
 

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FWIW I have an 02 with 3.55 gears and 285/75 16's and it gets about 20 unloaded at 65. I don't know what a 12v would get, but you could buy a lot of diesel fuel with as much as it would cost to regear.
 

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My 02 got about 18 mostly back roads around town with 3.55's.

Never got a mileage calcualtion on the highway cause I always had something behind the truck on the highway
 

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Discussion Starter #4 (Edited)
FWIW I have an 02 with 3.55 gears and 285/75 16's and it gets about 20 unloaded at 65. I don't know what a 12v would get, but you could buy a lot of diesel fuel with as much as it would cost to regear.
The electronic 24v trucks seem to get a lot better mileage in my experience than the older 12v trucks. I'm figuring that I'd be into a gear swap for roughly $500 if I did the work myself using brand new gears in the rear and a used set in the front. Considering that I can't ever do better than 15 mpg if I increase my speed to 65+ mph I would *guess* that I could probably make up the investment in about a year, assuming that I can get 19 mpg at 65 mph.
 

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With light to no load, it seems the biggest factor on Cummins fuel milage is where in the powerband you're operating. I have an '01 24V with 3.55's that used to ride on 265's. I went with 285's last time around and it dropped the RPM just enough that it's lugging the engine a little more and fuel milage has suffered some. It seems like such a little change that it wouldn't be worth messing with, but I have contemplated regearing to 3.73's. Doing so would bump the RPM right back up to where it was with 265's and the fuel milage should return.

I drive an average of 20,000 miles per year, so even a increase of 2 miles per gallon would result in a savings of over $700 in one year at today's fuel prices. It wouldn't take long to recoupe the cost of regearing.
 

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I had a 95 12V with 3.55's, 265 tires, intake, exhaust, 60hp injectors. The best it got on the interstate cruising at 67mph was 22mpg. It averaged 19. I couldn't ask for more but I ended up with 2 trucks and had to sell one and the Dodge sold first. I wish I still had it.
 

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With light to no load, it seems the biggest factor on Cummins fuel milage is where in the powerband you're operating. I have an '01 24V with 3.55's that used to ride on 265's. I went with 285's last time around and it dropped the RPM just enough that it's lugging the engine a little more and fuel milage has suffered some. It seems like such a little change that it wouldn't be worth messing with, but I have contemplated regearing to 3.73's. Doing so would bump the RPM right back up to where it was with 265's and the fuel milage should return.

I drive an average of 20,000 miles per year, so even a increase of 2 miles per gallon would result in a savings of over $700 in one year at today's fuel prices. It wouldn't take long to recoupe the cost of regearing.

So what rpm in your opinion is the efficiency range in a 12v? I've been experimenting but not come to any good conclusions. I'd like to go up to 315/75/16s from a RPM point of view but there aren't many that are load e in that size. Perhaps regearing to 3.73's might be the answer.
 

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Discussion Starter #8
With light to no load, it seems the biggest factor on Cummins fuel milage is where in the powerband you're operating. I have an '01 24V with 3.55's that used to ride on 265's. I went with 285's last time around and it dropped the RPM just enough that it's lugging the engine a little more and fuel milage has suffered some. It seems like such a little change that it wouldn't be worth messing with, but I have contemplated regearing to 3.73's. Doing so would bump the RPM right back up to where it was with 265's and the fuel milage should return.
That's the problem I'm having. The motor seems to be waaaaaay out of its powerband at anything more than about 60-65 mph. It's not that I can't cruise at 60 mph to get the mileage I want, but I think that I can get the mileage I want at a higher speed with some gearing changes. I tried tires first, but jumping up to 285s from 265s didn't drop my cruise rpm enough to make any realistic difference.

I had a 95 12V with 3.55's, 265 tires, intake, exhaust, 60hp injectors. The best it got on the interstate cruising at 67mph was 22mpg. It averaged 19. I couldn't ask for more but I ended up with 2 trucks and had to sell one and the Dodge sold first. I wish I still had it.
That's great to know! There is hope! :D
 

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My truck will do 17-18 in mixed driving and it has a 2-3" lift and the aerodynamics of a barn. I cruise 65-68 on the freeway turning about 1800-2000 rpms.
Travis..
 

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It takes a lot more energy to push the truck down the road at 65 mph. I doubt gearing is going to make a significant difference.

RPM isn't the only thing that changes with speed. The only real way to increase fuel mileage is to reduce engine load.

I changed from 31 to 35 inch tires and saw a very minor increase in fuel mileage, maybe 1.5 MPG.
 

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put bigger tires on, it will do the same thing but it will be cooler!!! unless you need your truck lower. my old 98 12v had 3.54's and 35s were fine empty, but towing it lacked BIG TIME. so i ran 285's cuz they were a good mix of height to drop my cruising rpm's down as someone already said to 1800-2000 while still having guts to tow.

my 06 has 3.73 and 33's and i LOVE IT. i put 35's on and hated it...
 

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Discussion Starter #12
put bigger tires on, it will do the same thing but it will be cooler!!! unless you need your truck lower. my old 98 12v had 3.54's and 35s were fine empty, but towing it lacked BIG TIME. so i ran 285's cuz they were a good mix of height to drop my cruising rpm's down as someone already said to 1800-2000 while still having guts to tow.

my 06 has 3.73 and 33's and i LOVE IT. i put 35's on and hated it...
I've thought about tires, but I really don't want to increase the height of my truck any more than it already is. Besides, I'd need new tires and a lift which would cost more than regearing.

My truck seems to get excellent mileage at ~1800rpm, which is kind of my target goal for where I want to be at 65-70 mph. When I start nearing 2000 rpm or higher the mileage drops and I can drive as fast as I want with no change in economy. From the way people are talking, I think the regear will get me what I am looking for.
 

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Unless you are towing super heavy, 4.10s are overkill on a 12v without running 35" tires. Put the 3.55s in and enjoy.
Travis..
 

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My 24v has 3.55's a 5" lift and I run 295/75/16's on it right now. So far my best MPG was 17, average is 14.98 and I drive slow. It has a crappy chip in it right now,so that might be part of the problem, that and a big winch bumper on the front.

I noticed absolutely no change between 235/85/16's, 295/75/16's and 315/75/16's except for how well it tows my 5th wheel. Otherwise it gets the same mileage whether I drive 55 or 85.
 

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Yeah, I have a 97 12v with a DTT AT, 285/75/16s, 4.10s and a "few" mods. Cruises at around 1800RPM @ 60mph with no boost showing on flatish terrain unladen. No issues towing anything but the mileage is not great when empty (17-18 mixed). I get 15 mpg or so towing my 16 foot tandem with 4 quads or the buggy on it and my old camper (approx 2200 lbs loaded). Does about 10psi at 60ish same RPM as above. I've upgraded camper size and don't really expect that to change much though as it never seems to make much difference how much weight is behind it. I do hate the 3-4 gearing span in the AT though. OD isn't low enough for big hills and 3rd is too low so you end up revving way to high or slowing down to 50-55 (I guess not a bad thing, I'm usually still passing most in the pass lane). That's why I was thinking about the gear change or upping the tire size a little, to get 3rd to be a little more rpm friendly and have better unladen mileage. I'd really like to go with rickson's and 285/70/19.5s though... just a lot more $$$$ to throw into the rig for how much gain (not just looking at fuel mileage here, as the performance/drive ease is also a big factor. This is just a tow rig, occasional 2nd vehicle and toy generally)? Any thoughts? Is it money down the drain?
 

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i run 315/75/16s on my 98 12v, I switched to 4.10s a month or so ago and I have went from 14-15 consistantly to 20-22 consitantly @ 65mph. I run t around 1900rpm so its a bit high, but it works well and I even get decent milage towing my xj around 14 ish so im pretty happy woth the change.
 

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With light to no load, it seems the biggest factor on Cummins fuel milage is where in the powerband you're operating. I have an '01 24V with 3.55's that used to ride on 265's. I went with 285's last time around and it dropped the RPM just enough that it's lugging the engine a little more and fuel milage has suffered some. It seems like such a little change that it wouldn't be worth messing with, but I have contemplated regearing to 3.73's. Doing so would bump the RPM right back up to where it was with 265's and the fuel milage should return.

I drive an average of 20,000 miles per year, so even a increase of 2 miles per gallon would result in a savings of over $700 in one year at today's fuel prices. It wouldn't take long to recoupe the cost of regearing.
Years ago, when I worked for Chrysler at the main Tech Center (and yes, it was chrysler at the time, 1997 I believe), there were some complaints of mileage not being what was expected with the 3.55 CTDs.... the recommendation from engineering, which I don't think was implemented (they were moving to the 24v motors anyway at that point) was to eliminate the 3.55s and make 4.11s standard, since our testing showed the 4.11s outperformed in MPG on the test trucks in real world situations.
 

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Years ago, when I worked for Chrysler at the main Tech Center (and yes, it was chrysler at the time, 1997 I believe), there were some complaints of mileage not being what was expected with the 3.55 CTDs.... the recommendation from engineering, which I don't think was implemented (they were moving to the 24v motors anyway at that point) was to eliminate the 3.55s and make 4.11s standard, since our testing showed the 4.11s outperformed in MPG on the test trucks in real world situations.
Depends on the overdrive ratio. My truck SCREAMED from the factory with 3.73s. The 6 speed NV5600 trucks that I drove with 4.10s were nuts...constant shifting.
 

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Depends on the overdrive ratio. My truck SCREAMED from the factory with 3.73s. The 6 speed NV5600 trucks that I drove with 4.10s were nuts...constant shifting.
Again, this was with 12v motors and the deep overdrives of the 4 speed auto, which is, what 0.67 or so?
 

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I have a 96 12v with 4.10 gears. I know my mileage is around 16-17 with slow highway driving (never much over 65) I thought about re-gearing but settled on tires and rims. I wanted to go to 19.5s anyway so I swtiched over and went with a 34.6 in tall tire. It picked up my cruising speed which was nice since I was getting run over at 60. I get about the same mileage at 70-72 now.

Towing I get about 12-14 depending on how fast I go and how much boost I'm running. With taller tires the 4.10s are nice with stock size tires I was always running around 3k to keep up with traffic

-ben
 
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