Swapping from 3.73 to 3.42 Gears

[DPKetchum]

On the Ford forums the guys are screaming NO to 3. 55 gears being standard. They say they DON"T want to pay for a option like 3. 73s and the trucks won't "WORK" with 3. 55s. Mileage went down cause truck works harder to move I would assume.

 

[BHaden]

Well I must say at this point there must be something terribly wrong as my milage actually went down. On repeated trips the milage has decreased by almost 2 MPG rather than increased. The truck seems to be running happier at the lower RPM's but the fuel economy tells a different story. It seems like the ECM figures any running around the city is excessive idling and goes into regen as soon as there is any sustained highway speeds. Also we have been hit with a bad cold spell and lots of snow and the milage also has taken a tumble, never had a diesel do this before.

I can see where lots of snow being pushed around by the tires and cold weather would affect mpg. Maybe a couple more tank fulls will give better info.



There may be something going on here that I have noticed with just about every gasser vehicle I have owned for the last 20 years. At about 75-80 mph, they all get better mileage than if you slow down 5 or 10 mph. I believe the manufacturers tune the computers to pass the EPA smog test which is done well within the typical "legal" speeds we all normally drive. If you speed up to slightly above typical highway speeds the computer gets "happy" and allows better timing and fuel settings and you end up with better mileage. I had an 89 Toyota Camry 2. 0/5 speed that got 33 mpg at a steady 65-70mph.

If I kicked it up to 80-85 it would get 38-40 and it felt a lot happier at that speed. 89 BMW 525i, same thing; 23-24 at steady 65-70 and 26-27 at 80+ and a much nicer running engine. You have effectively slowed the truck down with the gear change. I don't know if what I've seen on cars of the past is applicable to a late model diesel or not. Maybe it's just not a good thing on a DPF-equiped truck. No telling what surprises lurk in the new ECMs.

 

[Cattletrkr]

No change is needed with this year truck,the tire size will change it but not the gear ratio.

Bob

... because the speedo runs off a revs per mile counter on the axle right?

 

[Bob4x4]

That is the sensor being used now,rear diff tone ring.



Bob

 

[MrFixit]

Perhaps we are not getting all the MPG #'s because the #'s are not any good. We have seen one user getting less than 1 mpg maybe. Where are all the numbers? Not saying this won't work, there was just so much talk about this and a handful of guys trying this. I am curious as to the outcome.



Thanks

 

[Dbonzi]

SSSHHHHH!!!!! MUMS the word :-laf

 

[WOT]

The problem is that the fuel mileage increased so dramatically that the oil companies have put those members that did the swap on the watch list. They are all afraid of what will happen if they speak :-laf

 

[zstroken]

I can see where lots of snow being pushed around by the tires and cold weather would affect mpg. Maybe a couple more tank fulls will give better info.



There may be something going on here that I have noticed with just about every gasser vehicle I have owned for the last 20 years. At about 75-80 mph, they all get better mileage than if you slow down 5 or 10 mph. I believe the manufacturers tune the computers to pass the EPA smog test which is done well within the typical "legal" speeds we all normally drive. If you speed up to slightly above typical highway speeds the computer gets "happy" and allows better timing and fuel settings and you end up with better mileage. I had an 89 Toyota Camry 2. 0/5 speed that got 33 mpg at a steady 65-70mph.

If I kicked it up to 80-85 it would get 38-40 and it felt a lot happier at that speed. 89 BMW 525i, same thing; 23-24 at steady 65-70 and 26-27 at 80+ and a much nicer running engine. You have effectively slowed the truck down with the gear change. I don't know if what I've seen on cars of the past is applicable to a late model diesel or not. Maybe it's just not a good thing on a DPF-equiped truck. No telling what surprises lurk in the new ECMs.

You are correct about tuning the vehicle, why do you think the torque plate looks so goofy on a stock 12V. With a car you might see a mile per gallon improvement, but our trucks have such a frontal area, I have found that my truck gets best mileage around 55, you start going 65 and the mileage drops off. Around 55 I can see 20-21, 65 about 18, 75 about 17

 

[Bob4x4]

Just because the engine is quieter at low rpm does not mean it will be more eficient. All the 24vs have been more eficient at a slightly higher rpm than the 12vs. You will never get 12v mileage in a 24v. The higher the fuel prices go the more I drive my old 12v. :-laf



Bob

 

[MCummings]

I can see where lots of snow being pushed around by the tires and cold weather would affect mpg. Maybe a couple more tank fulls will give better info.

He said he's never had this happen before, so I figure he is comparing it to his past experience with other vehicles.

At about 75-80 mph, they all get better mileage than if you slow down 5 or 10 mph.

Not my '98 Durango. It gets 18. 5 at 64-65 and about 12 at 80. However, our Navigator doesn't drop so much, but over 70MPH the MPG drops about 1MPG every 2 - 3MPH.



BUT, I'm not taking into account that both those vehicles have a large frontal area, and "slipping through the air" wasn't on anybody designing mind. However, they are loaded up with lots of "cool".



Merrick

 

[courierdog]

There is also an animation by Cummins which I have seen but cannot find at this present time which also helps with the understanding of the Engine Break. Does any one know the link to it.
Thanks

 

[K5IP]

Sounds like an interesting swap. I too am of the opinion that you will not see anywhere near a 2 mpg increase.

Remember, power=torque X RPM X (a constant).

Therefore, if you reduce your engine RPM (for a given load) by 8. 3%, you will have to increase the engine torque by a corresponding amount. Engine output power, (and fuel consumption) will remain the same. The only effeciency change possible would occur only if you are operating the engine in a more effecient part of it's RPM range.

The older 12 valve engines had a peak effeciency at about 1600 RPM (cummins data). I don't think the newer engines have much of a peak anywhere in their normal operating range.

Sorry to rain on your parade, but you asked.

Rog

 

[seafish]

Hey all, this is an interesting and important thread. While I appreciate Courierdogs info on his gear swap, everyone reading should note that his info applies to his 6. 7 CTD with the 68rfe trans, not the 5. 9 with G56 manual or 48re auto set-up that this thread started on. I am pretty sure it's fair to say that the 6. 7 CTD is going to react differently to running at lower RPMs than the 5. 9 CTD.



So far, everyone has seemed to agree that the 3:73 to 3:42 gear swap would far more likely benfefit a 5. 9 CTD with the G56 manual than any other engine/transmission/axle combination. I am one of those who would eventually like to do this mod, but I was hoping to get more info and real numbers from people doing this mod on the 5. 9 CTD with G56 or 48re. Without causing any offense, can we keep this thread accurate to the 5. 9 forums??



With that said, does anyone have mileage info or other info to offer with this swap on the 5. 9/G56 combo??





TIA!!!

 

[dmclean]

rpm`s

there must be something wrong with my new 08-reg cab 4x4 6. 7 manual transmission with 3. 73`s-at only 60 mph I`m turning 2000 rpm`s in 6th (damn thing only gets 13-14 mpg not towing nothing!)

 

[twmcneilly]

It's more than just gearing; t's also related:



//www.associatedcontent.com/article/330356/how_much_power_your_car_needs_at_65.html?cat=27

 

[JWilmer]

DPKetchum,



You have some very good points. I bought a diesel truck to tow 5000 pounds with about 3000 miles a year. The rest of the 22,000 miles is with no load. I have plenty of friends who have gas Dodge trucks and the diesels get more than 50% better fuel economy. The time I tow empty, I would like to get the best mileage I can.



This board is full of products and ways that guys are trying to get better mileage. The way I am trying to is yet another way that nobody has done before.



This might be a poor example but here goes... . a new Corvette is a very fast and powerful vehicle and sucks fuel quite rapidly when driven hard, but can get 32+ MPG on the highway due to gearing in top gear.



You guys might be right. I may be totally fooled about RPM's and fuel economy, EGT's, etc. I just know that when I slow down, I get dramatically better fuel economy and it all can't be attributed to wind resistance.



Doing the 3. 42 gear swap might render my truck useless for towing. As someone else suggested, one could put smaller diameter tires on to bring it right back to the gearing it has right now for the 10% towing I do, which is all at one time.

I have a 06 MC with 3. 73/auto trans and 35" tires/2" lift kit... my new gear ratio = 3. 40's... I tow a 7500 trailer at 80 mph from MN. to WY. or ID. (snowmobiling) and at 80 mph (speedo corrected for bigger tires) my tach is right on 2000 rpm... with 33" tires it would hold OD for 1000 miles no matter the hill... with the 35" tires I had to "drive" the truck up the serious passes (no OD)... but short of about 20-30 miles of steep pass driving my truck didnt care even with a solid 20+ mph head wind across ND... .



I am even considering the 37/12. 5/17 BFG's (36. 3" tall = 3. 23 gear)... I too only tow for 2-4000 miles a year. . the rest is empty daily driving... the 37" would look sweet IMHO-LOL-BJ

 

[DPKetchum]

I've always had a truck to work and tow with. Very seldom do I or did use it for commuter vehicle to work or anywhere else for that matter. I've had over the last 20 years a number of good autos I bought used and put money into them but drove several hundreds of thousand miles. 1980 210 SL wagon,1984 Nissan Maxima,1985 Chevy Caprice Classic,2000 Mercury Gran Marquis,06 Mercury Gran Marquis. I bought all of these at wholesale or below and The Maxima,Chev and 1st Marquis I put over 100k on each. Not to say I didn't put some bucks into them getting them road worthy and keeping them maintained but ALL got/get over 20mpg mostly in comfort. The BEST was the Nissan Maxima. Inline 6 and 5 speed. Best driving one of the bunch. 28/32 mpg and put 150k on it. Bought it with 98k for $2500. A 4 door Z car. Ran,drove excellent with ice cold ac. Never thought of driving my diesel and one gas burner truck full time/daily even with cheap fuel.

 

[Tommyg]

I can't believe 16 pages of response to this question. Has anyone converted a stock 3/4 ton with G56 to the 3. 42? What was the result in towing?

 

[betterthanstock]

I can't believe 16 pages of response to this question. Has anyone converted a stock 3/4 ton with G56 to the 3. 42? What was the result in towing?

A G56 & 3. 42 would be nearly identical to nv5600 & 3. 73. But DC changed the later g56 to match the gearing of the nv5600. So which G56 are you talking about? 0. 78 or 0. 73 OD?

 

[ENafziger]

Has anyone converted a stock 3/4 ton with G56 to the 3. 42?

I just converted my G56 truck to 3. 07 gears! Well, kindof... I installed a Mitchell Gear Overdrive unit on Saturday. It offers 1-1 direct drive, or an 18. 2% overdrive, which when engaged, effectively changes a 3. 73 ratio to around 3. 07.

I'll probably post a new thread with results when I get a few miles on it. Right now, 70 mph is 1600 rpm in 6th over.

--Eric