engine/diff ratio combinations

April 15, 2005

I've seen threads discussing the 4.8L engne with the 3.42 rear differential and the trouble maintaining highway speeds on hills. My Yukon searches downshifts going over bridges and small hills. I believe I saw that the 4.8's with 3.73 diff's didn't experience that problem.

Input?

Is it possible that if I replace my 3.42 with 3.73 that I could actually get better mileage? I know the math doesn't work but when factoring in how much easier the engine would be working,,,,maybe.

Has anyone made this swap?

How did it work out?

Thanks,

P.S. The Westers tune helps (actually eliminated it when using mid-grade fuel) but I'd like to be rid of it completely.

496 CUBIC INCHES · April 15, 2005

If 3.73s will keep your truck from downshifting as much,I can see a MPG improvement.

What kind of MPG are you getting now? My brothers 2004 4x4 P/U gets almost 20MPG on the freeway. He has the 5.3 and 3.73s. Are you anywhere close to that?

snoman · April 15, 2005

If 3.73s will keep your truck from downshifting as much,I can see a MPG improvement.

What kind of MPG are you getting now? My brothers 2004 4x4 P/U gets almost 20MPG on the freeway. He has the 5.3 and 3.73s. Are you anywhere close to that?

<{POST_SNAPBACK}>

Boy this is a very touchy subject for some. To start with that truck should have never shipped with 3.42 and OD to begin with. They put that gear in because they can squeak out a extra 1/4 mpg on EPA dyno tech which does not really represent real world driving with hills and wind drag and such. Back in the old day when there was no OD in truck a 3.42 or 3.73 was a good compromize but OD changed all of that because even with a 4.10 rear axle ratio when OD is engaged, the overall ratio is about about 2.9 to one which is tall for a big SUV. I would not waste time going from a 3.42 to a 3.73, I would go to a 4.10. Your MPG will likely improve to because even though the RPM will increase some in a cruise the engine will be running much more efficeintly at a high vacum and in a RPM range that it tuning is more effective at too as the 4.8 does its best if allowed to wind up a bit. Even with this change you will still see 2000 rpm or less at 65 in OD and as a side benifit you will have a lot better performance even if you install bigger tires and OD will work much better too.

April 15, 2005

If 3.73s will keep your truck from downshifting as much,I can see a MPG improvement.

What kind of MPG are you getting now? My brothers 2004 4x4 P/U gets almost 20MPG on the freeway. He has the 5.3 and 3.73s. Are you anywhere close to that?

<{POST_SNAPBACK}>

I generally set my cruise between 75 and 77 (about 1900 to 2000 rpm) while on the hgihway. Funny but it seems to make a difference even with only a 100 rpm change. My mileage is between 16 and 18 at that speed depending on how many hills I encounter. Last Christmas driving up through the mountains in VA, KY and such it was 15 tops. And man it was shifting constantly. I was wishing I had a tranny temp guage (even in the winter) to see if I was hurting something.

snoman · April 15, 2005

If 3.73s will keep your truck from downshifting as much,I can see a MPG improvement.

What kind of MPG are you getting now? My brothers 2004 4x4 P/U gets almost 20MPG on the freeway. He has the 5.3 and 3.73s. Are you anywhere close to that?

<{POST_SNAPBACK}>

I generally set my cruise between 75 and 77 (about 1900 to 2000 rpm) while on the hgihway. Funny but it seems to make a difference even with only a 100 rpm change. My mileage is between 16 and 18 at that speed depending on how many hills I encounter. Last Christmas driving up through the mountains in VA, KY and such it was 15 tops. And man it was shifting constantly. I was wishing I had a tranny temp guage (even in the winter) to see if I was hurting something.

<{POST_SNAPBACK}>

4.10 gearing would yeild about about a 2300 to 2400 rpm cruise at those speeds in OD which will put the engine in a more efficent power range for the size of engine, size of load and the speed because at those speed s and loads now you are actually lugging you engine and making it work hard below its best RPM range. The RPM's I suggest are nothing for a 4.8 and youshould in a increase in MPG especailly in the terrain you talked about and it would likely never shift out of OD either. I have a 89 4x4 burb that I have had since new with 29" LT street tires and 3.73 gears and a 350 TBI which has a lower RPM range than your engine and more low speed torque too and when I went to Tenn a few weeks ago, I was really booking down and back and was well above 70 most of the time and saw 80 a time of two. It was very hilly and it only left OD once and I got just under 17mpg going down and I was loaded and truck has 170K on it too. I ran 31's on it for many years but it sucked on interstate hills sometimes and at high altitudes in the rockies were I go from time to time so I went back to 29's until I regear to 4.10 in the future. When I ran BFG AT 31 x 10.50's I never got over 15.5 MPG on cross country trip and most of times closer to 14mpg but have teased 18 mpg with current tires if I keep it at about 65 mph and gears and I corrected the speedo for both combos. Your truck should have atleast 31's on it now (or bogger) and 4.10's would be the ticket especailly in your terrian)

99-2door · April 15, 2005

Florida gulf coast probably isn't the most mountainous terrain.

You go to 4.10s there you are gonna see your MPG plummet. Either stick with the 3.42s or a 3.73. Your yukon is 2wd right?

snoman · April 15, 2005

Florida gulf coast probably isn't the most mountainous terrain.
You go to 4.10s there you are gonna see your MPG plummet. Either stick with the 3.42s or a 3.73. Your yukon is 2wd right?

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Yes it is not mountainous but anywhere north is. You will not see MPG go down, it will go up. People such as yourself is stuck on thinking that a 3.42 with w 4.8 and OD (effective OD ratio of about 2.3) is a great MPG combo. But then maybe you like have your tranny hunt on grades all of the time too. I guess I am old fashsioned because I like all of my gears usable all of the time, even on hills. I say a 4.10 because a change for 3.42 to 3.73 is hardly worth the expense and effort for noticable returns where with 4.10 you will really feel the difference. If this was a non OD vehicle I would not recommand a 4.10 for his use but that is not the case here. When he is in OD it is like a 2.9 in drive and that is plenty tall for that small motor and while 4.8 run good they do not have the torque of the bigger motors and need the gearing even more to do their best. And finaily, a 4.10 will reduce tranny strain/loads by about 20% accross the board and run cooler and last longer and better town MPG because it will get above the stall speed of converter quicker and slip less which means less fuel wasted to heat and TC stall slippage in town.

krayniac · April 15, 2005

Florida gulf coast probably isn't the most mountainous terrain.
You go to 4.10s there you are gonna see your MPG plummet. Either stick with the 3.42s or a 3.73. Your yukon is 2wd right?

<{POST_SNAPBACK}>

Yes it is not mountainous but anywhere north is. You will not see MPG go down, it will go up. People such as yourself is stuck on thinking that a 3.42 with w 4.8 and OD (effective OD ratio of about 2.3) is a great MPG combo. But then maybe you like have your tranny hunt on grades all of the time too. I guess I am old fashsioned because I like all of my gears usable all of the time, even on hills. I say a 4.10 because a change for 3.42 to 3.73 is hardly worth the expense and effort for noticable returns where with 4.10 you will really feel the difference. If this was a non OD vehicle I would not recommand a 4.10 for his use but that is not the case here. When he is in OD it is like a 2.9 in drive and that is plenty tall for that small motor and while 4.8 run good they do not have the torque of the bigger motors and need the gearing even more to do their best. And finaily, a 4.10 will reduce tranny strain/loads by about 20% accross the board and run cooler and last longer and better town MPG because it will get above the stall speed of converter quicker and slip less which means less fuel wasted to heat and TC stall slippage in town.

<{POST_SNAPBACK}>

krayniac · April 15, 2005

hope this helps

i swapped out my 3.73's for 4.56's in my 03 1500hd, 6.0, 305/ 50/ 20 average milage before was 9.5 - 10. after the gears i average 11.-12.5 no other mods were done after the gears. my theroy is that the eng/tranny are working less to move the mass. i cruise about 70/75 on the hwy. i did the swap for performance not mpg's, but it was an was a nice surprize. now, the cost of the install was about $1700.00 , so it would take some time to re-coop that. keep that in mind .

good luck

Wingnut · April 15, 2005

If 3.73s will keep your truck from downshifting as much,I can see a MPG improvement.

What kind of MPG are you getting now? My brothers 2004 4x4 P/U gets almost 20MPG on the freeway. He has the 5.3 and 3.73s. Are you anywhere close to that?

<{POST_SNAPBACK}>

I generally set my cruise between 75 and 77 (about 1900 to 2000 rpm) while on the hgihway. Funny but it seems to make a difference even with only a 100 rpm change. My mileage is between 16 and 18 at that speed depending on how many hills I encounter. Last Christmas driving up through the mountains in VA, KY and such it was 15 tops. And man it was shifting constantly. I was wishing I had a tranny temp guage (even in the winter) to see if I was hurting something.

<{POST_SNAPBACK}>

Speaking from personal experience, if you're going over 75, 4:10's will kill your mileage by at least 2mpg unless you're towing or driving through the Grand Tetons.

3:73's with an updated Westers would probably help, but to me...The 16-18 mpg you are currently getting is pretty dang good for 75 mph. If I go 75-80 (5.3 with 3:73's and 285's, which should reduce my final ratio to something close to yours), I get more like 15mpg, and mine probably weighs a bit less than yours. Of course, I don't have a tune, so that's probably where you're getting the couple extra mpg's.

April 16, 2005

Actually the tune didn't help my mileage much. I was at 15.8 avg before and 16.3 after. That's partly why I was thinking of a gear change (that and the feel better factor). $1700 is way more than I was expecting. Not sure what dream I woke up from but I was thinking maybe $300 installed with used parts. $1700 would never pay itself back.

99-2door - yea, it's 2wd

99-2door · April 16, 2005

15-16 seems a little low in a 2wd, but I always got around 17-18 in my 2wd reg cab trucks. Your Yuke probably weighs a little more than they did, so that may be about right. Gears are tricky because there are so many things to take into account....driving style and grades being a big part of it. Its hard to get a feel for what you really need without driving those gears over a period of time.

Now, I like deep gears as much as the next guy, but I think they have their place too. 4.10s are way too deep for a 2wd passenger vehicle.

Now of course everything I have just said is my OPINION. That should go without saying, but if don't somebody will reply with poor subject/predicate agreement and tell me I don't know what the hell I'm talking about.

krayniac · April 16, 2005

007fl

i paid $1700. for both front and rear diffs. , yours should be half that . sorry i didn't make it clear that mine is a 4x

snoman · April 16, 2005

Speaking from personal experience, if you're going over 75, 4:10's will kill your mileage by at least 2mpg unless you're towing or driving through the Grand Tetons.

3:73's with an updated Westers would probably help, but to me...The 16-18 mpg you are currently getting is pretty dang good for 75 mph. If I go 75-80 (5.3 with 3:73's and 285's, which should reduce my final ratio to something close to yours), I get more like 15mpg, and mine probably weighs a bit less than yours. Of course, I don't have a tune, so that's probably where you're getting the couple extra mpg's.

<{POST_SNAPBACK}>

Lets see my 2000 K3500 with a 5.7 does about 16mpg around 70 to 75 cruise on the few trips it has been on. It has 31's and weights about 5800lb without a driver so but your logic it will get 18 MPG with 3.73'? Not going to happen. It is a pipe dream and you are giving this guy false hopes. It takes HP to move that truck at 75 and the engine has to make it and it will deliver the best power (HP hour per gallon of gas) when operating at it peak VE (Volumetric Efficency) which also happen to be when peak torque is made too which is not at 2000 or 2200PM with a 4.8!!! (the 4.8 is a great engine but not a stump puller) It will take likley 50 to 70 HP to cruise that truck at 75 and more on hills and no magic tune is going to make that engine make a lot more power at low RPM.

Years ago I took a 1 month trip cross country in a 31 foot motor home with a 454 and a 4.56 axle ratio and no OD and it weighted about 14k lbs. (it did have some big 17.5" wheels and tires on it that where about 33's overall) I got about 9 in the mointains on highways and 11mpg on the flats over a 7000 mile trip. According to the gear logic of some I should have gotten terrible mileage but I did not and it would not have been better with taller gears either because the combo allowed the engine to make and use its power efficently for the load it had. The gearing allowed it to hold speed surprizing well on some grades too

WyRam · April 16, 2005

My 4x4 CrewCab 5.3 w/4.10s will do 18.2 to 18.5 at 68mph and 17.2 to 17.5 at 72 to 75 mph.4.10s won't plumet your mileage,these numbers on on fairly flat traveling at 4 to 5000ft.