Swapping from 3.73 to 3.42 Gears

[klenger]

Given any specific combination of speed, weight, frontal area, grade, altitude, and frictional forces, the HP required to move the mass down the road are the same, regardless of RPM. Therefore, if you want to reduce your RPMs, you must increase the torque to acheive the same HP. The big question is given any RPM/torque requirement, are you operating below, in, or above the RPM sweet spot.

 

[brods]

Not totally true. The oil pump, water pump, transfer case etc consume more power the more rpms they turn. Most of the following quotes are from here: http://www.mitsubishi-fuso.com/en/technology/technical_info/05_02.html?b5

However, increasing the rpm reduces the combustion efficiency and increases mechanical loss.

I think BSFC has more bearing than RPM when pulling a load (having the engine heavily loaded). The graph I saw for a 24-valve engine put the best BSFC at 1950 - 2000 RPM, but I don't know how it compares to a HPCR motor (don't have that one in front of me)... ... .

Vaughn and Jwashburn appear to be on the right track.

In general, engine fuel consumption depends greatly on the combustion efficiency and often has an inverse relationship with the torque curve. ... ... ...

In general, the engine rpm values that produce maximum combustion efficiency and maximum torque are nearly the same.

How this applies to part throttle cruising I have no idea, but there has to be some correlation. So once the rear gears are optimized for highway cruising, the next question is how will the gears effect acceleration and around town mpg? Will the lower rpms hurt torque converter efficiency when its not locked up? How will the gears effect the transmission shift points? Will the town mpg drop?



Dherbeck, maybe try giving Cummins a ring and ask an engineer their thoughts before cracking the diff covers. If you can live without 4x4 for a bit, have you considered just doing the rear and see how you like it before doing the front (if you go this route pull the front driveshaft so you can’t accidentally use 4x4 and break something)? Good luck and keep us posted!

 

[AKaiser]

Not totally true. The oil pump, water pump, transfer case etc consume more power the more rpms they turn. Most of the following quotes are from here: http://www.mitsubishi-fuso.com/en/technology/technical_info/05_02.html?b5

Well, the HP requirements at the road are the same, but the efficiency of the engine is reduced as a result of friction losses due to the factors you mentioned (HP used by the engine to turn the engine). Looking at the link you provided, it answers the mail as I figured. Note that in the chart, BSFC closely correlates to the torque curve, and begins to increase at about 2000 RPMs in this particular chart. We need to see the BSFC v Torque v HP curve for the engines we have, but I suspect the correlation is close.

Vaughn and Jwashburn appear to be on the right track.



How this applies to part throttle cruising I have no idea, but there has to be some correlation. So once the rear gears are optimized for highway cruising, the next question is how will the gears effect acceleration and around town mpg? Will the lower rpms hurt torque converter efficiency when its not locked up? How will the gears effect the transmission shift points? Will the town mpg drop?

If you have a G56, it's more a factor of raising the OD cruising speed, which would possibly be better served by replacing the . 79 ratio No. 6 gear with a . 74 gear. The whole problem for the G56 is the transmission in the 05 and 06 at least are not geared right for Interstate cruising speeds of 70-75 MPH. I changed the gear speed calculator for the G56 w/3. 73s found on klenger.net so 6th was . 74 instead of . 79, and that makes for 68 MPH at 2000 RPMs instead of 64 MPH. I added my 33 inch tires, and it goes to 71 at 2000. Put 35s and you get 75 at 2000 RPM. Tell me that won't increase your MPGs. I'd rather put the . 74 ratio 6th gear in than mess with the axles. For a 3/4 ton, at least, the . 74 seems like a better combination anyway.

 

[Chris and Sam]

ratio change

I would be very interested to see how it is going to work out. Cost is marginal compared to a us gear or gear vendor. My truck has the 3. 73 ratio and G56 trans. More potential mileage would be a plus. From what I have seen in the past, the best fuel mileage is at peak torque.

 

[AKaiser]

I would be very interested to see how it is going to work out. Cost is marginal compared to a us gear or gear vendor. My truck has the 3. 73 ratio and G56 trans. More potential mileage would be a plus. From what I have seen in the past, the best fuel mileage is at peak torque.

Right, and the torque curve is fairly flat from somewhere around 1800 RPM to in the high 2000s, so the best cruising speed RPM would be the lowest RPM that provided max torque AND the minimum necessary HP to keep the truck moving and not lug the engine. HP burns fuel. Increase the HP output, and fuel usage increases.

 

[betterthanstock]

Guys, a more realistic comparison would be to go at a lower speed, where wind resistance is not a big factor. Let's say you go at 30-40 mph on cruise control for one mile at 1800, then change gears to get 2200 rpm on cruise, and keep speed again in the same range 30-40 mph. Reset mpg gauge after you are at steady speed (both times), and compare mileage for yourself.

 

[TowPro]

Has anybody looked into the gear change in the trans? Is 6th gear on the output shaft in the back of the trans? This would also require changing the counter shaft gear, it is a removable gear or the complete counter shaft?

 

[Ereynolds72]

The "G56 Top Gear RPMs at Interstate Speeds" Problem

That's what I meant. That's indeed a perceived problem and not a real problem. I really want a 6spd manual but I do a lot of highway driving and the high rpms will keep me from buying a G56 unless a higher 6th gear is available or the 3. 42s are available for the axles. Putting the 3. 42s in the axles though affect all gears and not just the highway cruising gear.

 

[AKaiser]

That's what I meant. That's indeed a perceived problem and not a real problem. I really want a 6spd manual but I do a lot of highway driving and the high rpms will keep me from buying a G56 unless a higher 6th gear is available or the 3. 42s are available for the axles. Putting the 3. 42s in the axles though affect all gears and not just the highway cruising gear.

I'm guessing you meant that the G56 OD gearing is a real, not perceived, problem.

 

[AKaiser]

Guys, a more realistic comparison would be to go at a lower speed, where wind resistance is not a big factor. Let's say you go at 30-40 mph on cruise control for one mile at 1800, then change gears to get 2200 rpm on cruise, and keep speed again in the same range 30-40 mph. Reset mpg gauge after you are at steady speed (both times), and compare mileage for yourself.

MPGs at 30-40 MPH is not the issue. MPGs at interstate speeds where the wind resistance IS a factor is the question. Sure, that test would be a good indicator of what the BSFC curve is, but you'd have to drive a couple hundred miles each time and manually calculate the MPGs to get an accurate figure. Find me a test track somewhere, and I might be willing to run the test. Mine will run 22 on the O/H, but I know it's inaccurate. I also know that a steady cruise at 2000 RPMs at 60 gives 19 MPG versus about 17 MPG for a 75 MPH cruise at around 2400 RPMs.



The BIG question is will the wind resistance at Interstate speeds eat enough of the HP up on faster gears that the gain from swapping the OD to . 74 or the diffs to 3. 42 is minimal at best.

 

[BBowers]

Let me tell you, I have 35 12. 50 17 Dick Cepek FC II tires which gives me a 13% decrease in engine rpm. I have seen only a dip in fuel economy. I used to get about 21. 5 mpg on the overhead with 33 12. 50 17 procomp at's at about 60 mph steady. Now I am lucky to get 20. I know the tires weigh more and have more rotational inertia, but at a steady speed that inertia acts like a flywheel to KEEP you rolling! I am scratching my head still about why mileage went down so much. I expected maybe getting about the same mileage, but I think maybe I will eventually go back to at least a narrower tire.

 

[Turbo_Bob]

Let me tell you, I have 35 12. 50 17 Dick Cepek FC II tires which gives me a 13% decrease in engine rpm. I have seen only a dip in fuel economy. I used to get about 21. 5 mpg on the overhead with 33 12. 50 17 procomp at's at about 60 mph steady. Now I am lucky to get 20. I know the tires weigh more and have more rotational inertia, but at a steady speed that inertia acts like a flywheel to KEEP you rolling! I am scratching my head still about why mileage went down so much. I expected maybe getting about the same mileage, but I think maybe I will eventually go back to at least a narrower tire.

It is the larger cross section of the tires (wind resistance, drag) which lowered your mpg, the weight only has a little to do with it.

 

[brods]

Bbowers, every tire will have different rolling resistance. Maybe Dick’s tires in general have more rolling resistance than Pro Comps. Do they have a more aggressive tread pattern? Add to that the heavier rotational mass of the larger tire and it’s a MPG looser for sure. (Adding rotational weight is much worse for mpg than the same amount of static weight).



betterthanstock, imo it comes down to this: Will the motor will be more efficient with fewer, but more powerful, combustion events per minute. Any meaningful tests have to be done at the horsepower level where the increased efficiency is desired. What Dherbeck is seeking is at what rpm does the ISB most efficiently produce a specific amount of horsepower. The specific hp amount is what it takes to move his truck at 65 mph, if I understood his first post right, and calculates out to around 56 to 57 hp using the information from this site: http://mb-soft.com/public2/car.html

I get 2 MPG increase in MPG slowing 5 MPH in speed. The 3. 42 gears will slow my engine about 150 RPM at 65MPH, which would be like driving 60 MPH. I am hoping to get 2 MPG increase driving 65, turning 1650 RPM, same as if I was driving 60 now. I might not net all the increase, but expect to get close to 2 MPG increase.

Crunching some numbers and assuming the same 30% efficiency for the motor at both speeds, 65 mph = 17. 6 mpg 60 mph = 19. 6 mpg. So on paper this 2 mpg increase appears to be almost entirely from reduced aerodynamic drag from slowing down, not lower engine rpm. Doesn’t mean the gears won’t help, just don’t count on 2 mpg from a 150 rpm reduction. I’d try to get in touch with the experts at Cummins or Dodge before doing anything.



Akaiser, I agree with you that dropping in a higher (numerically lower) od gear is the way to go if possible. Do bear in mind that Dherbeck’s truck is an auto, so unless a higher od gear is available for the auto (doubtful), he is stuck with changing axle gears and all the pros and cons of doing so.

 

[AKaiser]

Akaiser, I agree with you that dropping in a higher (numerically lower) od gear is the way to go if possible. Do bear in mind that Dherbeck's truck is an auto, so unless a higher od gear is available for the auto (doubtful), he is stuck with changing axle gears and all the pros and cons of doing so.

Which is why I placed "G56" in bold. My comments are strictly aimed at the G56 transmissions. I think the other trannies with 3. 73s are probably set up at optimal overdrive ratios as-is.

 

[AKaiser]

It is the larger cross section of the tires (wind resistance, drag) which lowered your mpg, the weight only has a little to do with it.

I think it may be partly a function of RPMs being too LOW. When you put 35s on a 48RFE, you get fourth gear speeds of 60 @ 1500 RPMs.

 

[Turbo_Bob]

I think it may be partly a function of RPMs being too LOW. When you put 35s on a 48RFE, you get fourth gear speeds of 60 @ 1500 RPMs.

Ya, the sweet spot is 1600 RPM's where peak torque starts.

With a stick shift trans all one needs to do is go in 5th gear at slow speeds.

But with the auto trans one could turn off the overdrive, at least with my 96 Dodge had that feature, a push button.

 

[DMork]

I have been looking at the exact same logic as you have. I have an 06 6Spd G56 and the final ratio spins your engine faster than the auto. My problem is I have a 4X4 and to do this, I need to sets of R & P. $1200 minimum. Taller tires will not help your mileage from what I have seen. Your engine will run slower, but you are pushing more air. Typically, I have seen economy degradation in most engines when running above the sweet spot RPM's. With a 5. 9L, you should not be continously spinning 2100 RPM's. It just does not sound right. 1900 would be great for 70 MPH.

________________________________

'06 2500 4X4 Megacab 3. 73 G56 6 SPD

 

[MLisle]

Any update on the 3. 42 gear install or a new final gear for the G56????

 

[TowPro]

Any update on the 3. 42 gear install or a new final gear for the G56????

Last week I started a thread about the new final gear for the G56. Looking at the service manual, it looks like the counter shaft or cluster gear is one complete assembly. Changing 6th would require replacment of this assembly and I bet its way over priced. 6th gear is not in the rear of the trans, its inside. This would be a complete tair down.

 

[APersondek]

FWIW..... We used to have an old 3/4 ton full size Chevy van with 6. 2L Detroit and the infamous 7R400 4spd auto transmission with 3. 42 gears..... it got the job done when running empty or with family and gear packed up in the van... . though once you got say 5~6000 lbs behind with our flat bed utility trailer both the engine and transmission would run noticeably hotter... . I never did hear why Dodge went away from 3. 55 gears... . I think 3. 73 is too tall and 3. 42 is just a bit too short.....