Swapping from 3.73 to 3.42 Gears

[seafish]

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

Looking forward to getting your results and impressions; Why did you choose the Mitchell unit??

 

[Tommyg]

Mine is an early 2006. I think it still is . 79. Where did you get the Mitchell?

 

[ENafziger]

Looking forward to getting your results and impressions; Why did you choose the Mitchell unit??<!-- google_ad_section_end --><!-- google_ad_section_end --><!-- / message --><!-- sig -->

I'd been looking around a long time for an overdrive unit that would help with fuel economy, and reduce engine speed/noise/longevity. Everything I researched seemed to lead to the Gear Vendors unit or U. S. Gear.

I really liked the U. S. Gear model, but they don't make anything to fit the G56 transmissions. The Gear Vendor unit cannot be used while an exhaust brake is engaged.

I ran across this Mitchell overdrive unit, and it had everything I wanted. It's rated for 35,000 lb GVWR, and is a synchronized gear-to gear unit. It's internals are very robust, and it's enclosed in a heavy aluminum case with cooling fins built in. It can be used with the exhaust brake. It may be placed in either direct (1-1) or overdrive. The specific overdrive ratio of this unit is perfect for splitting each gear if so desired (no gear overlaps with the overdrive unit), and shifts similarly to the transmission.

While Mitchell is still in business, they don't currently offer anything for light duty trucks. I was able to find this one for sale by an individual, and it had been completely rebuilt by Mitchell only 700 miles ago.

I'll take some pictures soon...

--Eric

 

[raychem]

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!!!

My post #209 should help you. But I did not change my gears for milage... I did it B/C crusing at 85 turning 2500 rpm was too much for me.



Richard

 

[seafish]

Thanks Richard, it's been so long since Iv'e read this thread, I'd forgotten that someone had actually posted some results for the conversion; Are you still happy with the way your truck drives around town, in the country (hills), as well as on the highway?? TIA

 

[seafish]

I'd been looking around a long time for an overdrive unit that would help with fuel economy, and reduce engine speed/noise/longevity. Everything I researched seemed to lead to the Gear Vendors unit or U. S. Gear.



I really liked the U. S. Gear model, but they don't make anything to fit the G56 transmissions. The Gear Vendor unit cannot be used while an exhaust brake is engaged.



I ran across this Mitchell overdrive unit, and it had everything I wanted. It's rated for 35,000 lb GVWR, and is a synchronized gear-to gear unit. It's internals are very robust, and it's enclosed in a heavy aluminum case with cooling fins built in. It can be used with the exhaust brake. It may be placed in either direct (1-1) or overdrive. The specific overdrive ratio of this unit is perfect for splitting each gear if so desired (no gear overlaps with the overdrive unit), and shifts similarly to the transmission.



While Mitchell is still in business, they don't currently offer anything for light duty trucks. I was able to find this one for sale by an individual, and it had been completely rebuilt by Mitchell only 700 miles ago.



I'll take some pictures soon...



--Eric

Eric, thanks for the update; sounds like a great unit; Looking forward to pix and impressions.

 

[raychem]

Thanks Richard, it's been so long since Iv'e read this thread, I'd forgotten that someone had actually posted some results for the conversion; Are you still happy with the way your truck drives around town, in the country (hills), as well as on the highway?? TIA

Yeah... . no complaints on driving as a DD with a silver bullet for a turbo and a very tight converter. I will still smoke 1 2 and third with ease. Its flat here in Florida except fot the northern parts of the state but I have been from FL to RI all along the east coast and the hill and mountains have never been a problem. As for the hwy, the truck feel like my old 12v does. The truck will still give me 18 plus on the hwy at 80-85 mph. A little better if I slow down but thats hard. . lol





Richard

 

[WOT]

Bumping... No G56 guys out there with MPG feedback yet? Someone has to have done the swap by now!

 

[BHaden]

Bumping... No G56 guys out there with MPG feedback yet? Someone has to have done the swap by now!

Is it possible all the pre-07 G56 guys are all waiting for "someone else" to go first??

 

[WOT]

Is it possible all the pre-07 G56 guys are all waiting for "someone else" to go first??

Yep... sure seems that way. Now if I can get everyone to chip in on some for me I'll give it a try first!

 

[wstmmd]

I had my o8 switched from 3. 73 to 3. 42- both front and rear axles- apparently the 3. 43 is used in many Gm prcducts. best mileage before was 17. 1-325 miles Az to NM- best after-20. 2 Reno to Eugene,Or- makes the hiway mileage better and acceptable to me. wstmmd

 

[wstmmd]

Sorry =I meant 3. 42 In GM - its apparently the same product used in the recent 08 and 07 Diesels. wstm