Rear End Gear Change - COMPLETE!

[sbdaigo]

I just picked up my suburban from the shop, just had 3:42 installed in the rear end. Thanks for all the info here on the site. My RPM

[DmaxMaverick]

Sounds great.

If you have a T400 tranny, you are right on.

However, if you have a 4L80E (which you should), read on....
Your RPM's with 3.42's and 245's should be closer to 1800-1900 RPM's in OD at 65. Your tach or speedo has to be off. If you (or anyone) has changed the alternator, it could have the wrong pulley on it. The tach get's its signal from the alternator.

[sbdaigo]

Well, I do have the 4L80E and I had my alternator changes long ago, I wonder if this is correct? Now I went from happy to sad. Any more info? Anyone else have this set up - 3:42 with 4L80E? What are your RPMs? I seem to be on the same mark as others with other people with this set up - and the speedo works right so they must have put in 3:42

[catmandoo]

what did you have for gears to begin with.my 94 sub 3/4 would turn about 2000 at 55 and 2500 at 70,and i'm running 4:10.and 265 tires.think maybe your alt pulley is smaller!!

[JohnC]

Most likely the alternator pulley is wrong. Do a search: I think the correct size is posted somewhere here. Or have someone give you an RPM reading from a Tech I or II to compare to the tach.
Don't be sad: get it corrected and the numbers will make you even happier!

[catmandoo]

ya if you dropped 400 rpm at 65 thats a heck of an improvement,makes me want them 3;42 THAT MUCH MORE IF I COULD GET DOWN TO 2000 AT 70 THAT WOULD MAKE ME HAPPIER THEN S#*T,I SEE WEST COAST GEAR SHOWS A 3:21 RATIO,IF THATS TRUE MAN YOU COULD GET IT DOWN TO SAY 1700 RPM AT 70 MAN YOU COULD PULL SOME MILAGE THERE BABY.beings were on the milage subject i pulled 27 day before yesterday,think i found the secret,keep the egt below 400 across the board and man pulled just about 26 yesterday and was into it alittle harder and today it's looking like another 26+ day oh by the way its on my 92 c1500 extcab shortbox with 3:08 gears.banks turbo mandrel downpipe.and monday i was doing 70-75 on the i-state for 2/3 the trip.

[markrinker]

I haul heavy loads and also 'haul ass' when driving. 4.10 gears and 285's. 10mpg. Uphill, downhill, 10.

[Basshopper]

I have 3.42 and the 265 tires at 75 mph 2000 rpm and 65 is 1850 to 1900. Just upgraded the tires and the rpm came down about 150 rpms overall my speedo from the factory had always read 4 mph fast and with the larger tire it put my speedo right on with my gps. Did not loose any pulling power that I could tell even towing my bass boat in the mountains. Was a good investment to upgrade my tires.

[computer monkey]

I have 245/75 16 with 3:42 gears so far I like them only after doing the mods to the Suburban.

Water injection added a month ago.

[moondoggie]

Good Day!

Our Sub has 3.42 gears - 30 mph/1000 rpm. In other words, 60 mph ≈ 2000 rpm, 65 mph ≈ 2165 rpm. I do NOT have stock size tires, but have corrected (math, not VSSB) for that in the above. This is wrong - corrected in subsesquent post.

Blessings!

Brian Johnson, #5044

[ 08-26-2004, 08:55 AM: Message edited by: moondoggie ]

[Marty Lau]

[quote]Originally posted by sbdaigo:
[b] Well, I do have the 4L80E and I had my alternator changes long ago, I wonder if this is correct? Now I went from happy to sad. Any more info? Anyone else have this set up - 3:42 with 4L80E? What are your RPMs? I seem to be on the same mark as others with other people with this set up - and the speedo works right so they must have put in 3:42

[moondoggie]

Good Day!

Speed vs. rpm? You might want to look at my 9 Mar 04 post in Speed vs RPM with 4.10 rearend (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005936)(Click on the colored text.). I should have gone back here before trying to remember what rpm's my Sub turns. 2000 rpm @ 60 mph is my pickup, which has an effective 3.936 overall ration when you factor in the 4% taller tires (1500 rpm @ 60 mph with the Gear Vendors Aux OD engaged).

Assumptions (for your truck):

* 3.42 gears
* 4L80-E AT (0.75 OD)
* 245/75R16 tires (30.47" OD, 95.72" circumference)


rpm @ 60 mph = (60 mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (95.72 inches/tire rev) X (3.42 driveshaft rev/tire rev) X (0.75 engine rev/driveshaft rev)

rpm @ 60 mph ≈ 1698 rpm
Then, rpm @ 65 mph ≈ 1839 rpm

If you switch to 265/75R16 tires (31.65" OD, 99.43" circumference):

rpm @ 60 mph ≈ 1630 rpm
rpm @ 65 mph ≈ 1766 rpm

Yea, I need to get out more. :(

Blessings!

Brian Johnson, #5044

[ 08-26-2004, 09:17 AM: Message edited by: moondoggie ]

[cruzer]

I believe the correct pully diameter is 62mm where the belt rides or 2.44".IIRC

[sbdaigo]

I measured my pully and it is 2.44

[cruzer]

Are you sure it's going into o/d? Is the torque converter locking up? Did the shop give you your old gears back? You can pull the rear end cover and do the math to double check the ratio. Did you measure the actual diameter or the tire. You can find "rpm calculators" by doing a search on the web and enter the values that pertain to your vehicle.

[moondoggie]

Good Day!

I'm fairly, but not completely, sure that the OD ratio for our 4L80-E's is 0.75. Therefore, if you were cruising in 3rd gear instead of OD:

2264 rpm @ 60 mph;
2452 rpm @ 65 mph.

If you were shifting into OD gear, but the TC (torque converter) wasn't locking, I don't think you'd see this high of rpm unless you were pulling something heavy - from reading posts here for a couple years & my personal experience, it sounds like you would expect ≈ 200 rpm higher at these speeds with the TC not locked. Fortunately, it's REALLY easy to determine if the TC is locking or not. (Review: Cruise at 65 - 70 mph WITHOUT using the cruise control; tap the brake pedal lightly, enough to unlock the TC but not apply the brakes; with properly operating TC, rpm will increase ≈ 200 for a couple seconds, then drop back down when TC re-locks.)

Blessings!

Brian Johnson, #5044

[JohnC]

[quote]Originally posted by sbdaigo:
[b]So I wonder what is up. At 70mph my tach reads about 2350RPM - nowhere near these 1900

[DA BIG ONE]

4.10 to 3.42 in my 4wd burb has been one of the best mods yet. Runs just over 1,800 rpms @ 65 mph, and 2,000- @70 mph.

[Keith Richards]

Could someone tell me the cost of the gear change?I'm either keeping the 4.10's and getting a lift,or changing to taller gears.If I ever make some $$$. :rolleyes:

[catmandoo]

i found the gears for 239.00,as far as what someone will charge you is another story.

[sbdaigo]

at 65MPH with cruise control on, my tach is at 2200 - so I wonder if the gears were 3.73 that they put in instead of 3.42. Anyone have 3.73 gear tach reading? Thanks, Rino

[DmaxMaverick]

Time to stop guessing. Your numbers still aren't adding up. Even with 3.73's, it's still off a little, but a lot closer than with 3.42's.

Go out to your truck. Take a jack with you, and a white crayon, tire marker, or chalk. Take a helper with you if you have one. He/she only needs to watch and count. You can do it yourself, but 4 eyes (glasses don't count) are better than 2.

Chock the front wheels, so it won't try to roll on you.
Jack up one wheel, so it can rotate freely. Put gear selector in any postition other that P. release E-brake.
Chalk a line on the ground, and a matching one on the tire.
Chalk a mark on the drive line, somewhere that it will be easy to accurately count rotations and see from right in front of the wheel you have jacked up.

Slowly rotate the wheel twice and count the rotations of the driveline.
You are reducing the shaft rotations of by having only one wheel up. The reduction will be 1:2, meaning the shaft will rotate at 1/2 the rate as it would if both wheels were rotating together.

If you have 4.10's, 2 rotations of the wheel will rotate the shaft approx. 4 turns.
If you have 3.73's, 2 wheel rotations will turn the shaft approx. 3-3/4 turns.
If you have 3.42's, 2 wheel rotations will turn the shaft approx. 3-1/2 turns.

It is necessary to use 2 wheel rotations. If you only do one, the driveline marks will be too close to know for sure which gear set you have. The gear differences will be approx. 3/4 turns apart with 2 rotations, vs. 3/8 turn with one rotation.

If you have a limited slip diff, you will have to jack up both wheels, and double the shaft rotations. Example, for 3.73's, 2 turns of the wheel will turn the shaft approx. 7-1/2 turns. Or, one rotation will yield the actual gear ratio of 3.73:1, or approx. 3-3/4 shaft turns for each wheel rotation.

Now you will know, without a doubt, which gears you have in there.

Don't forget to put it back in park and set the E-brake before letting it down.

If all this doesn't satisfy your needs, you will have to remove the diff. cover and count the teeth and do the math. Divide the pinion teeth into ring gear teeth.

Edited to correct the math. My apologies.

[ 08-27-2004, 01:48 PM: Message edited by: DmaxMaverick ]

[DA BIG ONE]

Originally posted by Keith Richards:
Could someone tell me the cost of the gear change?I'm either keeping the 4.10's and getting a lift,or changing to taller gears.If I ever make some $$$. :rolleyes: Broward Power Train in FT. Laud. did my truck back in Apr, cost a scad under $1,600 for front and rear gears, install kits, setup & synthetic gear lube.

The biggest pain in the as_ was trying all the aftermarket adjustment units to calibrate the VSSB, only to find that none of them would work on my truck because of my T-cases Automatic on demand 4wd w/its 3 speed sensors. Finally, I did the hack myself to the VSSB, but keep in mind that if I did not have access to this board and all the help of its members I would have been SOL for a long time.............

[catmandoo]

dmax i'm kinda confused by your figuring,if for one turn of the tire the shaft rotates 8 1/4 times for 2 turns it would be 16 1/2 ,which would be some stump pullers from hell.for the posi unit. yes jack both wheels off the ground and count shaft turns for 1 rotation of the tire,you could also do this for the one wheelers but if something slows one wheel or the other you get a false reading thus only jack one and double the rotations to take up for the spider gears. if the shaft turns just over 4 turns for 2 of the tire it's 4:10.for the 3:73 the tire would turn 1 and 3/4 turns and for the 3:42 it should turn just under 3 1/2 turns.

[sbdaigo]

OK, got all this information and here is something that just went across my pea brain. I did the math for my VSSB adjustment which meant that I had to put into the equation the tire size, gear size of the rear end, and yada, yada. So, my speedo is correct (with someone following me) so then it must be that my tach is WRONG. I will recheck the speedo today, but I believe somehow that the tach is reading this wrong. I check the pulley on the alternator, seems to measure correctly - so I will keep you posted.

By the way, you guys are really great here. Thanks for sharing your brains!

Rino

[JohnC]

The pulley for the alternator is identified by it's outer diameter, which is measured at the largest point, i.e. the edge of the pulley. The correct pulley is 2.6"

Confirm you are getting OD and TCC lockup. Going down the road at 65, right foot on the throttle, touch the brake pedal. If the torque converter is locked, it'll unlock. Pull the shift lever from ODto 3. If it's in OD, it'll shift down.

If all else fails, the tooth count should be engraved on the edge of the ring gear.

Or, get a ride with a Tech II (did I mention that?) :cool:

[ 08-27-2004, 08:48 AM: Message edited by: JohnC ]

[sbdaigo]

JohnC,

My pully messures 2 ? from the edge to edge - maybe that is the issue if you say 2.6?

[JohnC]

How big is 2.? ? AFAIK, 2.6" is the largest pulley available for the CS130 alternators. Anything smaller is going to make the alternator spin faster and the tach read higher.

If you can't find the right pulley, I have access to them, but for a one off the shipping will kill the deal. If there's a real need, I could get a couple, but someone whose core business is more aligned may already have them.

[DmaxMaverick]

catmandoo

You made me go back a re-read my post. You are correct. Did my math backwards. Oops....

I fix it.

Thanks

[sbdaigo]

John, my computer tried to put 3/8 and made a different symbol, so it measures 2 3/8. Sorry about that.

Rino

[catmandoo]

is anybody here running 3:21 in these 14 bolt floaters.i see west coast gear offers one,that would sure improve the mpg numbers on the interstate but i would think the tcc would have to be recalibrated to kick in later ,at 45 it would be turning maybe 1200 or so.i'm after milage as i might pull the lawnmower trailer around once in awhile so i'm not to worried about that.i'd like to be turning in the 1600 range at 75.

[Marty Lau]

Originally posted by catmandoo:
is anybody here running 3:21 in these 14 bolt floaters.i see west coast gear offers one,that would sure improve the mpg numbers on the interstate but i would think the tcc would have to be recalibrated to kick in later ,at 45 it would be turning maybe 1200 or so.i'm after milage as i might pull the lawnmower trailer around once in awhile so i'm not to worried about that.i'd like to be turning in the 1600 range at 75. Hey Catmandoo;
I'm not an expert but when I went from 3:73 to 3:42 the TCC lock up stayed at the same RPM it was the speed at which the lock up occurred is what changed so mine now lockup at about 1500-1600 rpm about 52-55 mph. I would think with 3:21 lock up would be between 60 to 65MPH? That's in overdrive. With stock tires 3.21 gears should give you 60mph at 1600 rpm. 75 you should see about 2000 rpm vs about 2540 RPM @75mph with 4.10's To get 1600 rpm at 75 you would need a 2:58 ratio with stock tires.

[ 08-30-2004, 08:18 AM: Message edited by: 16ga SxS ]

[Marty Lau]

Originally posted by sbdaigo:
at 65MPH with cruise control on, my tach is at 2200 - so I wonder if the gears were 3.73 that they put in instead of 3.42. Anyone have 3.73 gear tach reading? Thanks, Rino at 65 MPH with stock tires and 4L80E transmission you should see the following RPM with the various rear ratio's
3:42 = 1830 rpm
3:73 = 2000 rpm
4:10 = 2200 rpm
this assumes correct VSSB for tire and gears.

Rino I would not think the shop messed you up on the ratio but check by jacking up the back of the truck and counting the tire turns per turns of the drive shaft. I would guess it's your pulley or tach problem and the VSSB not corrected, you said the truck is much quieter which is a good indication they did change the gears and did you get the old gears back? When I changed mine I got my old ones back incase I wanted to go back to 3:73's. do the timing for the a messured mile hold 60 mph incicated should take 60 seconds any variation will show how much your off.

[ 08-30-2004, 08:01 AM: Message edited by: 16ga SxS ]

[catmandoo]

well i have 4:10 and it turns right at 2000 at 55 and get up to 65 and i'd say by the tach 2350 to 2400 and i have 265 tires instead of the 245's so it should be lower,so somethings amiss here,but there is a rebuilders tag on the alt so the pulley could be wrong,i'm gonna have to go measure tomorrow.

[moondoggie]

Good Day!

The only way I'm aware of to be sure of rpm vs speed is to use math, as in my 26 August post. If you plug in the right numbers, you WILL get the right answer. :D The only difficult number to find is the OD of a given size tire, which was actually provided somewhere on the Page a couple years ago, in a spreadsheet that I somehow now have on my computer. When I've used the equation in my 26 August post, it's always given the right answer within 1%. Why guess?

Blessings!

Brian Johnson, #5044

[Marty Lau]

Hey Brian I use the calculators on www.4lo.com (http://www.4lo.com)
they have the calculator for the gear and transmission also a calculator for tire size to get that figure. It's pretty handy.

[moondoggie]

Good Day!

Kinda figures I'd find the hard way to do it