Changing rears

[6.5 Suburban]

I have a 1996 2500 series Suburban LT with 145,000 miles. Runs and rides perfect. Trouble is, it has 410 rears. I do not use this truck for towing anything, except maybe a light duty landscape trailer with a quad or 2. The rears are much heavier than what I need. I was debating changing the rears to a 3:73 or 3:42. Mostly, I'm hoping to improve my gas mileage and make it more useable for highway travel. The RPMs are around 2300 at 65 mph, and I get 16 mpg at best at that speed. I am desperate! I do not want to get a mini van! I love my truck and it is pefect for my wife, 2 kids, and 3 dogs. Anyone have the lower rears in their truck? What type of mileage do you get? Anyone ever change their rears?
Thanks

[Chevylover]

If you have a 4WD, don't forget to change the front too !


Cu,
Sven

[hraney]

I have a 4.10 in my 97 Crew cab K3500. I am averaging a solid 12mpg. I chalk that up to a 4x4 truck that weighs 7100# empty.
BTW, I put pencil to paper and w/ my (paid for) truck getting 12mpg and my wife's new (financed) Honda Pilot getting 20mpg it will be 7 years before her Pilot is as cheap to operate as my truck, even factoring in some major repairs to the 1 ton.

[6.5 Suburban]

I got a price of about $1500- $2000 to do the rears (front and rear). So figure $2500 in reality. Buying another car for commuting is going to cost me at least $4000 for something roadworthy and reliable. Never mind insurance and maintenance. I'd rather change the rears. I need to know if going to a lower rear is going to give me a substantial enough mpg improvement to make it worth it. I own the truck, so it may be cheaper to change them. Anyone have lower gears???????????
Mike

[3bals]

I have a 98 2500 TD pickup with 373s and I had a 94 TD pickup with 342s. I didn't notice any difference in mileage or RPM from one to the other. I would have ordered 342s on the 98 also, but they weren't available that year in a 2500 4x4 diesel. I think I'm running about 2300 rpms at 75 mph. I'll check for sure. Oh, and I have a manual trans.
Jody

[6.5 Suburban]

Thanks. What kind of mileage do you get? I have an automatic trans., but it should be close with the manual. What is IAFF? I'm Sprinkler Fitters Local 696 Newark, NJ.
Mike

[Garth J]

I have a 1996 2500 series Suburban LT with 145,000 miles. Runs and rides perfect. Trouble is, it has 410 rears. I do not use this truck for towing anything, except maybe a light duty landscape trailer with a quad or 2. The rears are much heavier than what I need. I was debating changing the rears to a 3:73 or 3:42. Mostly, I'm hoping to improve my gas mileage and make it more useable for highway travel. The RPMs are around 2300 at 65 mph, and I get 16 mpg at best at that speed. I am desperate! I do not want to get a mini van! I love my truck and it is pefect for my wife, 2 kids, and 3 dogs. Anyone have the lower rears in their truck? What type of mileage do you get? Anyone ever change their rears?
Thanks

check out www.gearvendors.com its an overdrive unit that fits behind the tranny with a lockout for 4 wheel drive I had one in my 85 3/4 ton with 4:10 it was great ...Garth

[3bals]

Thanks. What kind of mileage do you get? I have an automatic trans., but it should be close with the manual. What is IAFF? I'm Sprinkler Fitters Local 696 Newark, NJ.
Mike

I get about 17 mpg plus or minus depending on the load, how far my foot is into it and/or weather. I got 10 this last summer with a 11.5 ft slide in camper on board and pulling a 10 trailer to Utah and back. I've got as high as 19, empty on a warm summer day.
IAFF is the Internaional Association of Fire Fighters.
Jody

[94sub]

I get 18.5mpg all day long and I have 4.10's w/ automatic trans. With the family and gear ( 6 kids and wife) we still get 17 - 17.5 mpg. Towing a 32' travel trailer I drop to 12 but that's good to me.

[DragNasty]

I averaged about 16.5 mpg on the interstate running 80 mph and around 14 mpg in town. The only time that Ive towed (only had the truck a month) I got 11 mpg. That was pulling my 18ft trailer with a full size truck on it, running 75-80. That was also with no mods done to it yet. Im can live with that kind of mileage.

[94Mule]

Does your tranny have overdrive built in? If it does, how long ago since the fluid been changed?

[DragNasty]

Does your tranny have overdrive built in? If it does, how long ago since the fluid been changed?

Whats that got to do with fuel mileage or rpm range?

[rypie32]

What size tires do you got on it? Got a friend that has 285-75-16's with room to spare, I bet 305-70-16's would fit-they are like a 33-12.5. that would cut your gear down some......

ryan

[6.5 Suburban]

I change the rear and trans fluid regularly. I have OD. The drivetrain is in pefect order. It is doing what it's supposed to with 4:10s.

I checked out gearvendors.com. For a 2500 GM vehicle it's $2875 for their unit. It is very cool the way it works, but that cost is not practical for me. That is just the cost for the unit, not including shortening the driveshaft. It's worth checking out that site.

I have 285/75/16 also. My brother in law gave me a set of wheels off his 2005Silverado. My tire guy said any larger would rub the front fender under certain situations. Don't want to do a lift. The truck is pristine. I don't want to make any substantial changes to the suspension. Plus it is an every day truck and my daughter is 8. I don't want to have to lift her into it everytime.

I put an AFE cold air intake on about 3 months ago with vitually no increase in fuel mileage. Although it did improve the power. Sounds pretty cool too.

Now for the good news. I also have a 2000 Impala with 110k and the intake manifold that's going. I may buy my wife HER minivan to replace that instead. Either way it has distracted her nicely from MY Suburban. I don't think I'm going to change the rears. Not cost effective enough to spend the money. Plus like I said earlier, I don't want to make substantial changes to the drivetrain. It's just too nice of a truck!

I appreciate all the input, and will still check daily for tips. This website is great.
Mike

[John R]

I get about 17 mpg plus or minus depending on the load, how far my foot is into it and/or weather. I got 10 this last summer with a 11.5 ft slide in camper on board and pulling a 10 trailer to Utah and back. I've got as high as 19, empty on a warm summer day.
IAFF is the Internaional Association of Fire Fighters.
Jody

I just recently traded off my 1995 ext cab 4x4 3/4 ton 350 gas job with 3:73s i have done as well as 23mpg with it it all depens on weather, terrain, DRIVER ha-ha. I pulled a 27 foot 5400 lb fifth wheel camper trailer with it to alberta canada through hills etc. I live in maitoba canada,it averaged 9 mpg on my trip im sure i did better in manitoba and saskatchewan but alberta was the real test! steep grades, long steady grades you name it . I lugged the truck down to see what it would do ... down to 80 kph (in 3rd only not overdrive never tow with this year make of truck if you want too save your tranny;although it had the tranny cooler kit on it) and cruised at speeds up to dare i say up to 130 kph. only to see what it could do im sure it would have done more i thats as fast as i wanted to tow my camper.I wanted to know the capabilties of my truck.I since traded it off on a 1995 td crew cab long-box dually 4x4 more power ar.. ar.. ar.. plus i needed more cab room and a bigger box as well as towing capacity .

[njdevi11]

I recently read in a magazine (i think it was last month's four wheeler) about a guy who had a new f-350 that he drove on the high way a lot and wanted to get the most mpg possible. Among the biggest modifications i can remember from it were 3.07 gears, 220 degree thermostats, huge exhaust and intake and a custom fuel curve programmed into the computer, if he kept it under 2000rpms it would be frugal but when it got above that it opened the engine up.

Gears are not too expensive check out ebay or summit racing, depending on which gears you have now and which gears you want to go to it should not cost much more then $200/axle. If your looking for good fuel go straight for the 3.07's, worse case your transfer case probably has a 2:1 4-low, that would put your 4-low gears at 6.14 if you really need that low end power.


some gear carriers have different ranges, you might need a new differential carrier since i know for a dana 44 the split is 3.73- is one carrier and 4.10+ is another. Its possible to go from 3.73's to 4.10's with special "thick" gears. I'm not sure if there is a logical opposite called "thin gears." I believe on the dana 60 the split is at 4.10- 4.56+.

If you can do brakes you can do the gears yourself. All you need is the gears, a master install kit (includes new bearings, crush sleeve, shims and gear marker compound), the gears, a large 1"+ socket and an impact wrench (for the yoke). First remove the driveshaft from the yoke and use the impact wrench to remove the bolt holding the yoke to the pinion gear. Open the diff cover and drain, pull apart the brakes and pull the shafts and unbolt the 4 bolts holding the diff in. hold onto the shims and remember where they came from and go, this is a good starting point for when you have to put the new gears in. Unbolt the ring gear and swap it for the new one. One thing thats cool about a dana 60 full floater is that you can pull the axle shafts out without even taking the tires off.

watch powerblock on saturday morning on spike tv, i've seen almost every one of those shows go over installing gears on multiple episodes.

[midniteplowboyy]

I have all three, 3.42's, 3,73's and 4.10's, there's about 2mpg difference between each, on the highway the 3.42's might get 3-4mpg over my 3.73's, my 3.42 truck is a standard and the other two are 4l80's. They get around 16, 18 and 20-22mpg respectively.

I love the 3.42 truck, nice and quiet to drive, cant really tell its a diesel driving down the road and I've put 700 miles on a tank of fuel a couple of times, now if my D-max could do as good as my 6.5's empty I'd be happy, but it does do better on fuel towing a load.

I'm going to change my 4.10 truck to 3.42's, its just a 2 wheel drive (the other two are 4x4), I think it will pay for itself alot quicker than some guys think, not only in fuel saved, but also engine wear.

[Husker 6.5]

I recently read in a magazine (i think it was last month's four wheeler) about a guy who had a new f-350 that he drove on the high way a lot and wanted to get the most mpg possible. Among the biggest modifications i can remember from it were 3.07 gears, 220 degree thermostats, huge exhaust and intake and a custom fuel curve programmed into the computer, if he kept it under 2000rpms it would be frugal but when it got above that it opened the engine up.

Gears are not too expensive check out ebay or summit racing, depending on which gears you have now and which gears you want to go to it should not cost much more then $200/axle. If your looking for good fuel go straight for the 3.07's, worse case your transfer case probably has a 2:1 4-low, that would put your 4-low gears at 6.14 if you really need that low end power.


some gear carriers have different ranges, you might need a new differential carrier since i know for a dana 44 the split is 3.73- is one carrier and 4.10+ is another. Its possible to go from 3.73's to 4.10's with special "thick" gears. I'm not sure if there is a logical opposite called "thin gears." I believe on the dana 60 the split is at 4.10- 4.56+.

If you can do brakes you can do the gears yourself. All you need is the gears, a master install kit (includes new bearings, crush sleeve, shims and gear marker compound), the gears, a large 1"+ socket and an impact wrench (for the yoke). First remove the driveshaft from the yoke and use the impact wrench to remove the bolt holding the yoke to the pinion gear. Open the diff cover and drain, pull apart the brakes and pull the shafts and unbolt the 4 bolts holding the diff in. hold onto the shims and remember where they came from and go, this is a good starting point for when you have to put the new gears in. Unbolt the ring gear and swap it for the new one. One thing thats cool about a dana 60 full floater is that you can pull the axle shafts out without even taking the tires off.

watch powerblock on saturday morning on spike tv, i've seen almost every one of those shows go over installing gears on multiple episodes.
Are you talking about that red F350 with the front facia off of an Excursion so he could use the front air dam? That article was in Diesel Power Magazine, either Dec or Jan issue, can't remember. Anyway I have a 94 C2500 HD cab and chassis with an 8' steel utility box and standard cab. It has a 14 bolt 8 lug axle, and according to the stamped letter code, a 3.42 ratio. It has a 4L80E, and LT245/75R16E tires at 80 psi. On the Interstate the tach shows 2750 rpm at 83 MPH in OD with the converter locked(computer limited). When I bought the truck in fall 03 with 174K, I got 15 on the hiway. Right before it barfed both heads last winter at 224K, it was down to 13 MPG. I know the equation, but it's after 4am and I'm getting foggy, but it seems to me that my revs are too high for that ratio at that speed. I'm wondering if a previous owner switched to deeper gears? You all seem to be getting decent milage. See my trace for what I'm doing to my engine on rebuild.

[midniteplowboyy]

Husker, that sounds about like my 4.10 trucks rpm and milage, I think it turns about 2500rpm @75mph, my 3.73 truck doesn't have a tach so I really dont know what it turns, maybe I'll have my 4.10 truck on the road in a few days and I'll check it at 2750.

[94K30]

I average about 16.5 MPG around town, and just checked highway milage for first time, was 18 MPG coming back from NH yesterday ( @70-75 MPH ) my Milage has steadily inproved since I bought truck ( a few months ago) and have been repairing/improving things as they come up. The intake ( SSDiesel supply) helped, and even the lift pump/OPS showed a gain.....
Where can I look to see what my rear ratio is?

[grancito]

Look at the options sticker in the glove compartment, GT4 is 3.73, GT5 is 4.10, other info on "GM option codes G to M"

[94K30]

GT4 it is.... I am glad of that, on my Trailblazer, I made them get me one with 343's, all they had in stock were the 373's... after all, we have low range of you need short gears once in a while
Thanks Grant,
Marc

[Craig]

I get 16.5 to 17.5 regardless of city or highway. The 4.10's work good for city but do no good for highway. I plan to go with 3.42's real soon. Since I have a manual trans with a granny gear, I was wondering if 3.07 might be better. I also have 4 wd with low range.

[grancito]

There are conflicts in opinions of the advantage of gear ratios for fuel economy, I have found little difference with ratios, they mainly change wear on the engine, it takes x hp to move the vehicle at a given speed, so if you are near the economic fuel curve, real flat for a diesel, then the gear ratio changes little the fuel consumption. It is more important the efficience of the engine.

[torque454]

My black 96 4x2 burb makes 22mpg running 70 to 75mph. Thats with 3.42 gears, stock size tires, and 225,000 with the original injectors. :O New injectors may result in better milage. Those are in the list to-do soon.

[Diesel60]

dragnasty what kind of gears due you have to get that kind of mileage duing 80? i have 410 on my suburban 4x4 6.5 and still hard to get about 14-15 miles a gallon duing 65 ?

[njdevi11]

diesels work much better at low RPM and are more efficient, 2 stroke detroits ran at a whopping 800rpm.

When you get the revs too high on diesel engines your going to run into a situation where the fuel wont burn fast enough. Giving the fuel more time to completely burn and give you that continual low rpm torque that diesels are great for is going to give you more bang for your buck.

Keeping rotational inertia down and intake/exhaust restriction down will also help fuel economy.

(this is all what taller gears help you do)

[SCREAMINXL]

What 2 stoke Detroit was you around? The 6V92T that I have been around turn 2,400 RPM on the gov.

[Garth J]

The locomotive two strokes top out at 910 RPM 645 cubes per hole

[grancito]

The Mitsubishi 2 stroke engines at the power station here, turn at 100 rpm, and run on road tar, 45,000 hp.

[Garth J]

They must heat it first id love to see that thing running

[grancito]

They heat and centrifuge the tar and add 5% diesel. There are two of them and my packing plant 200 meters away vibrates. The engines stand 4 stories high, 12 cylinders with turbos. I haven't seen the new one, it's 65,000 hp.

[torque454]

Also a 3.07 and 3.42 will have a negative impact on the rest of the drivetrain. It is harder on the transmission, especially first gear. Grant and I have discussed the possibility of my 96 having some first gear problems (have a noise similar to that of a big bus or truck auto trans in 1st gear. Sort of a whining noise) that might be due to the high gears and being used to pull trailers. I am quite happy with the 3.42s tho. Im afraid youd be unhappy with the 3.07s..... lol. Id say my favorite gears would be 3.73s. But like i said i am happy with the 3.42s as well

[SCREAMINXL]

I wonder how much fuel they inject per cylinder per event. Even on road oil they must cost a small fortune to run.

[Garth J]

I was going to ask that same question

[grancito]

They are much more efficient than turbines, though very expensive to maintain, sometimes the rings break after one week from the high sulfur oil, and also crack the cylinder liners, but the upgrades are sorting out the problems.These are the first of such size in the world, so lots of warranty repairs.

[RmGuY0708]

I have a '94 2500 with 3.73's and average 17-18 mpg unloaded.

[sootjunkie65]

I get about 20-21MPG with my 3:42's I dont know what RPM im running because I dont have a tach. Anybody know where I can get one? im having some difficulty finding one.

[njdevi11]

The best tach's are the ones that count the teeth on the flywheel. You can get them from isspro but it will probably be around $250, the cheaper ones run off the alternator and still are a lot more expensive then tach's for gassers.

I found a faria Diesel tach for a boat on ebay for $20, I'm still trying to track down a sensor and information on how to wire it. i'm assuming it gets wired like an EGT Gauge I just need to find the sensor.

[94K30]

I get about 20-21MPG with my 3:42's I dont know what RPM im running because I dont have a tach. Anybody know where I can get one? im having some difficulty finding one.
aftermarket gauge companies, like ISSPRO, make diesel tach's. there is one type that has you glue on 2 magnets onto the balancer, and has a sensor that reads them....... Look at ISSPRO #R8503(0-4000 RPM), with sensor kit R8900 ( GM 6.2-6.5 diesel) PM if you need a link to find them online....
Marc

[Garth J]

I had a Isspro programable tach on my 6.2 the sensor was on the flywheel pan and it worked great

[Beach]

I have a 98 chev1500 4X4 automatic with 265 000K on it. Last year I installed a lowering kit. droped 4in in the front and 4in in the back and replaced the tires to 235/70/16 with a smooth tread pattern. I used to run 285/75/16 with a aggressive tread pattern. Fuel mileage went from 15mpg to 20mpg.
I just changed my gear sets to 342 from 370s on rear and front diffs and still run the 235/75/16's and my milage went to 25mpg on the average.
I think I can still do better by changing the thermostats aswell. All these changes help a little and I will see close to 30mpg yet!

[torque454]

How did that stuff actually help milage? It should have hurt it. And how do you lower a 4x4 (and why would you?) Something doesnt add up. Please explain.

[grancito]

Simple, the speedo now reads wrong.

[torque454]

Ahhhh damn you're right. I so should have thought of that. Theoretically that truck should be using more fuel than ever. Changing to a lower gear, and then changing to smaller tires thus lowering the effective gearing even more.

[grancito]

OK but changing gear ratios makes little difference to fuel consumption, I'll wait the flack for that statement, but it really changes little from my experiments.

[torque454]

I tend to agree, but the theory is that it would. Due to the higher rpms. I myself can see lower gears to some degree helping some, unless youre really getting on it or going excessivly low. Ive pondered changing from 3.42s in my 350 burb to 3.73s. Lower gears make it easier for the engine to move its load around, so it isnt working as hard. The harder it works the more fuel it will burn. BUT if you lower the ratio too much, the engine will spin it brains out and get little done and burns more fuel. I figure on my gas burb, the 3.73s will make the 350 work less to move that big tank around. Thats easier on the engine, and on the transmission and tcase. And, if i drive just a mere 2-3 mph slower, the engine wont be turning much if any faster anyways. The main place it would hurt would be on the highway and even then, just slow it down a mph or two. That is how *I* look at it. And if you think about it, it does make sense.

[Beach]

Ok I checked out my mileage again and I was wrong it isan't better than 22mpg. I am usually traveling all highway and at 60mph. I don't care what YOU say my odometer is correct (gps) . My truck has less rolling resistance with the smooth tread pattern tire. Smaller tires also make for less weight that has to me spun at the end of the drive. Lower vehicle to the ground equals less wind resistance (less wind going and dragging under the truck).
I don't know why anyone else would lower a 4x4 but I wanted to try this for improved milage and it has helped a bit.:)

[torque454]

No, i dont care what YOU say, it isnt correct if youve changed gears and tire sizes. That thing is way off and if your gps says its right on then you should toss in file 13 because its junk. The only way the odometer could be right is if you had the speedo recalibrated. You didnt say jack about that, just that you changed gears and tires.

[SCREAMINXL]

Everything you did had to help the fuel mileage on the hi way, unloaded not towing.

Last time I checked the IFS front end would be riding on or almost on the bump stops if you lowered it 4" in the front and would ride/drive like crap since you pretty much removed all the suspension travel and completely changed the geometry of the front control arms. If you didnt have the DRAC recalibrated your ABS light should be on all the time giving you a gear ratio error code and the tranny will also upshift and downshift early and late.

Enjoy your......urban 4x4.....i guess:rolleyes:

[Beach]

No, i dont care what YOU say, it isnt correct if youve changed gears and tire sizes. That thing is way off and if your gps says its right on then you should toss in file 13 because its junk. The only way the odometer could be right is if you had the speedo recalibrated. You didnt say jack about that, just that you changed gears and tires.

You don't care what I say? You are telling me how it is? I have the truck right here! Are you right here?!

Sorry If my GPS was out? Or has it occured to you by changing the tire diameter and the gear ratio they cancel them selves out! Is that saying Jack enough about that?
Your file 13!

I thought this was a place of discussion about ideas and tech info?

[torque454]

In that case the effective gear ratio is the same. Therefor it wont effect mileage at all. And the liklyhood of it being dead on and canceling out just 100% exact is slim to none. You can believe anything you want, but the numbers dont add up buddy. Stop foolin yourself. Enough said, i've just won this arguement.

[Beach]

Thanks for telling me who won. I wasen't sure who the ref was.

[torque454]

I did. Think about it... if you change tires and gears but the speedo was still dead on, you didnt change your effective gear ratio any at all. And the weight of the tire has very minimal impact. Im not sure youd notice it at all. So you ditched 20 pounds off your truck at a max. Im sure that made your truck get a billion times better milage. (sarcasm). Wouldnt even be enough of a difference to pay for itself.

[SCREAMINXL]

Well it looks like Beach's speedo is pretty darn close, not exact.....close. Lets review some facts.

285/75R16 = 32.8" tall, 3.73 Gear Ratio, .7 Overdrive, 64mph @ 1700RPM

235/70R16 = 28.9" tall, 3.42 Gear Ratio, .7 Overdrive, 61mph @ 1700RPM

So buy lowering the vehicle, reducing rolling resistance and still turning the same rpms the mileage had to improve.

Hate it when I'm wrong, but the computer wont set an ABS code with that small of a difference and I was wrong.

[torque454]

It may have helped a little but it couldnt be anything significant. Certainly not a (nearly) 50% increase in milage like he was initally claiming. I mean cmon, 15 mpg to 25 mpg. Thats a 10mpg difference, thats almost 50% change. And then he claimed he was thinking he could get 30. that IS a 50% difference. I say get real. If you could increase your milage by 50% just by doing that stuff, wouldnt everyone be doing it? Now, 25 is beleivable with a truck in perfect tune with stock tires and 3.42 gears. I get 22mpg with my truck and its in need of injectors as they are original. I have the 3.42s and only 2wd. But something isnt right, that stuff doesnt add up to 50% difference.

[SCREAMINXL]

I agree torque. Should see some mileage improvement but not to the point of needed to stop every 100 miles or so to drain the excess fuel off.:)

[94sub]

94 sub 2500 287,000 currently getting 19.5 with 4:10's. would the 3:73 gears help enough to warrant the cost?

[SCREAMINXL]

I would say no unless you get the parts for free or really cheap. 19.5 is pretty good mileage for a burb.