Fast355

You can think what you want, on the RV in question made a huge difference to turn a few more RPM. It was a Workhorse W22 with the 8.1/5spd Allison/4.56s. That may seem like a lot of gear but the RV chassis used 235/80R22.5" tires that are almost 37.5" tall. It had the Banks power pack on it which consist of a cold air intake, long tube headers, and performance oriented muffler as well as a hand held programmer. With the stock gearing it would not go into OD until 65 mph and would hold it down to 60 mph @ 1,750 rpm. At 70 mph it was turning just 2,050 rpm in 5th. Dropping down to 4th at 65 mph only brought it up to an anemic 2,600 rpm. On long grade it would drop to 3rd and unlock the converter, tach at 4,000 rpm. I am pulling the exact RPMs for the combo from a gearing calculator. Riding along in the RV with the data coming out of the PCM being monitored it was immediately apparent that it was undergeared for the weight and frontal area vs available torque. The owner wanting to pull a Jeep behind and improve performance unloaded, gave me the available gearing options and on my advice went with 5.69 gears. After 5.69:1 gears the RV was a completely different machine. It would climb slight grades and accelerate without downshifting. At 60 mph in 5th it was turning 2,200 rpm. At 70 mph it turns 2,600 rpm. Dropping to 4th at 70 mph on a long climb brought the tach to 3,600 rpm. Even with the Jeep in tow it never drops to 3rd. With the Banks Power Pack/5.69 Regear even towing the Jeep it gets better mileage than the RV managed to get pulling itself around when it was stock.

Fast355

If the OP were to post his tire size, I can put some numbers into the calculator for his setup and show where it lacks in gearing. 3.73s and tire size with the 4L80E are what is making his combination pudgy. Even the factory tow ratings are a good 2,000 lbs higher for the 4.10 gearing setup. GM even felt the need to give a 7.4 or 8.1 Express van a 3.42 gearing option. Tow rating for a 3.42 gear is only 6,500 lbs. With the 3.73 the rating bumps to 8,000 lbs. With the 4.10 gear its rated 10,000. I notice the pattern with this and the writing is on the wall.

Also remember the weight in the vehicle subtracts from what you should or can legally tow. If you have 2,000 lbs of payload in the truck, your max GCVW does not change, you take 2,000 lbs off the trailer capacity and vice versa. If you have 2,500 lbs of payload capacity and 600 lbs of tongue weight (10% of a 6,000 lbs trailer), you now have 1,900 lbs of payload capacity. In the scenario above the truck would be 100 lbs overweight despite being at the 8,000 tow limit. I am making an assumption that his tow rating is 8,000 lbs and being that he has 6 adults and cargo he is likely very close to it. Jayco may also state the tongue weight as 520 lbs, but it could likely be higher, just depends on how the trailer is loaded.

Fast355

Here is the information on the GM Heritage site for the Suburban, Yukon XL should be identical.

https://www.gmheritagecenter.com/doc...t-Suburban.pdf

Vindii

Not going to argue it anymore as nobody is going to change anyones mind. But I'll discuss if we can keep it sane.

In all fairness the RV your talking about is not really apples to apples to a stock yukon 8.1. RV was tuned with headers and CIA. That makes a difference. It also had a different trans.

What was the final result on the MPG? I'm very surprise if they drove around at 3600 rpm and got better MPG. But with gear changes and such that really a lot of changes to factor in. You didn't really just drop the shifter into 3rd and now you save fuel.

Again in fairness I live in the midwest. So we dont have huge grades around here. So the OP's experience will likely be different than mine. My truck has a cam and a tune also. But I dont think that give it more than maybe 30hp max if that. So not exactly same comparisons either.

Ive read that 3:73 gearing is 10.5K weight and 4:10 is 12K. 6.0L was 7.5 and 9.5 I believe.

Either altitude and grades make his experience drastically different than mine or there is something mechanically wrong with the truck. Maybe its pulling power for some reason that we dont know about.

James B.

iTrader: (1)

I'm going to respond to the original post and address a few concerns:

First, the 4L80-E (and 4L85-E that you have) is intended and designed to sustain maximum toque rating in overdrive. The overdrive planetary in these transmissions (identical 4-planet unit in both 80e and 85e) is enormous, and that is not accidental, so in that respect it is qualified to handle the load continuously.

Second, with regard to the constant downshifting, if that's happening drop it down a gear manually. Constant downshifting is a symptom of the engine being unable to make enough horsepower to provide the required transmission output torque to sustain speed. More horsepower is available at higher engine RPM and that's what's needed to pull the load. Constant up and downshifting between third and fourth accelerates wear of the overdrive clutches and should be avoided by manually downshifting and leaving it there.

Tuning - All 3/4 and 1 ton GM calibrations are configured to sustain and survive wide open throttle at any engine RPM. That means you can pull flat-out pedal to the floor without causing any damage to any component. It won't overheat a spark plug, a valve, an exhaust manifold, a catalytic converter, the transmission or rear end, etc... In order to make that possible the calibrations are severely conservative - low timing advance and rapidly diminishing timing curve with manifold pressure, and power enrichment after 60 seconds of sustained manifold pressure above 55kpa to keep things cool.

With stock tuning, if it's maximizing fuel economy while towing you're looking for, it needs to be kept under 55kpa of manifold pressure. Get a manifold vacuum gauge or use an OBD-II port tool and smartphone app to monitor it. Allowing the engine to spin up faster by dropping a gear while towing will help accomplish that goal.

Kerr

I didn't read all 6 pages, but.....

ok so rpms you posted you have 373 gears, the tires are different size but shouldn't matter..

In case no one mentioned it in the other pages, you have the 8.1L 496ci, not the wimpy brother 6.0L which is a total different beast ( we have both) the 8.1L makes ALL of its power at LOW rpms. so when it just barley turning the 2000 rpms, its probably making more hp and tq than 6.0L gas.

also keep in mind, you cant really hurt it unless you run out of oil, its iron heads, iron block meant to have a hard life, at any rate these are NOT RPM motor, so if its wanting to run at a low rpm its fine, just give it a little gas to hold it, also is your tow haul working? In comparison, our 6.0L pulls hills at 6000 rpms, on the ones that the 8.1L does actually down shift it stops revving at 43-4500 rpms.. but were talking 16% grades and stuff...

6MPG is not bad considering what your feeding, and towing...

towing will not hurt it in 3ed, if you need to lock it in, or just feel better with it in 3ed thats fine, just keep a eye on oil usage, these are still old school stuff really and big block love oil, if your run hundreds of miles at 3500 rpms its bound to drink it...

on a side note remember that these where made for women to tow horses... they are NOT perfect but your really have no other option... besides ford and diesel, and when the poop hits the fan the bill to fix a diesel you could buy another suburban, trust me, been down that road..

Other option would be check tow setting, up shift speed, down shift speed vs tire size and torque management... But to be honest unless the trailer is like a huge para shoot it should pull that weight all day at any speed you want..

CaptCameltoe

iTrader: (3)

tgui

Wrong thread!!!

Onetonfun

So I see a big argument about what gear to use and I say what does a gear have to do with the motor burning fuel. You want the least amount of input to the throttle as possible and things that affect that are restricted intake and exhaust systems. The sure fire way to make a difference is headers a free flowing exhaust and a intake. (Short headers build power lower in the rpm then long tubes.) The next time you drive pay attention to how much your adjusting the gas pedal in third vs fourth and that could tell you what is making your engine happy. The less pedal input means the engine isn't working as hard and should help the mpgs go up alittle.

2004silvy

Sorry wrong thread...