A Different Rear End Question

[spikesputnik]

Here's my dilemma. I have a 5.3l with 4l60, 3.42 rear end, 20" wheels. While this combination is fine for casual driving around, it does feel "overgeared" at times, such as going 40 mph in 4th gear at barely over 1000 rpm. If I had a 6 or 6.2, the better low end toque might handle it better, but this 5.3 doesn't lug that well( though once spun up, it rips). At the other end of the speed spectrum, say 75-80 MPH, I'm doing about 2100 RPM and feeling overgeared as it needs to downshift or unlock torque converter at the slightest grade change. Again, not enough motor for the gearing, or conversely too much gear for the engine.

While I have had a tune by Justin that made huge improvements in tranny performance and to a lesser degree throttle responsiveness, it is still not enough to overcome the hard numbers of 5.3 vs 4sp vs 3.42.

So I'm thinking of a 3.73, which I think would have me up to 2300 RPM on the freeway, maybe up and out of the "downshift zone" of the torque curve. Or a 4.10, but with RPMs then getting up to 2500 @80 mph....

With either rear end I'm afraid of how my mileage might suffer. Or might it not change proportional to RPMs- ie may not change much as it won't be struggling with the gearing??

Currently I'm getting 15-16 mpg freeway speeds( AFM is turned off) to 16-17 mpg easy backroad and city driving.

 

What to do, what to do?? I did have the pleasure of driving a rental '07 2500 2WD with a 6 spd a while back when mine was in for repairs. And while the truck itself was an abused turd, the engine was all torque/no zip (seemed to have other engine issues), the tranny was sweet-it had a gear for any situation!

So short of getting a 6 spd dropped in (for a few grand I'm sure), am I doomed???

[pm26]

Is this a 2WD or 4WD? If it has a 4 speed transmission on an '09 model , I assume that it is a 2WD extended cab. I would recommend either 3.73 gears, or simply go to 17" wheels and lower profile tires. The reduction in gearing will be proportional to percentage reduction in the diameter of the new mounted tires.

 

My '06 crew cab 4WD has 3.73 gears with a 4L60 transmission and at times I wish I had 4.10s. But I am in Florida and everything is pretty flat here. I have the stock 17" wheels/tires. I get about 15 MPG city and 17-18 MPG highway. No AFM on this one.

 

(I just noticed you are in Florida too.)

[66nova]

If you are not towing much, then 3.73s with your tire and wheel combo is a nice every day gear.

 

I think they would be perfect for your use.

[Starrman]

I have 3.73's in mine and my RPM's are 2050@70, 2200@75, and 2350@80 with a stock size 265/70/16 tire. I get anywhere from 16-18mpg on the highway (if I keep the motor under 2000rpm I get better milage), with the non flex fuel 5.3 and a bit more weight of the SUV.

[V8nuts]

I feel you pain as I am in the same boat. 3:42 to 3:73 is not that mutch of a differance I would say do the 4:10 gears and I have decided I will be putting the 4:10 gears in mine. Do you do a lot of heavy towing?

[spikesputnik]

V8ntus-I may be towing a 20' boat or 15 hand horse occasionally, but that'll be rare and not on the interstate. I can easily relax/slow a bit while pulling too.

Starrman- just about the same numbers I get, I guess because our tires ODs are the same despite different rims. Your narrower profile I'm sure helps your mpg, too.

 

Yea, it's quite annoying to be cruising 80mph with 400# load in the bed (on I-75 in Fla which is basically flat as pm26 sez- I live middle of the state and about 90' ASL) and the subtle grade change associated with an overpass necessitates it to kick down one. Before my tune it was much worse as it would hold the gear forever, lose too much speed, then drop twice and spin up frantically to 4500rpm in the process. Justin got it to unlock +/- shift earlier and more decisively, but the fact is it still needs to due to the lack of beans at 2100rpm. Now it doesn't do it more often, just executes it better.

 

What would a reasonable cost estimate be for changing the ratio to a 3.73, assuming new OEM parts but done at a Ma&Pa or small chain shop? The only reason I say new & OEM is that the rest of the truck has only 10K, so using "used" parts doesn't sound so good.

[pm26]

I see a bunch of 2009 Silverados and Tahoes with the 4.8 liter V8 and 3.23 gears, and heavy duty tow package. What kind of stupid gear ratio is that for HD towing? This has got to be the worst set up yet.

[deyomatic]

I see a bunch of 2009 Silverados and Tahoes with the 4.8 liter V8 and 3.23 gears, and heavy duty tow package. What kind of stupid gear ratio is that for HD towing? This has got to be the worst set up yet.

 

 

I drove one for a couple of years with this setup, a 2wd with no towing package...the acceleration was terrible.

 

I think the lower you go the better. Even if mileage suffers it won't be drastic given what our trucks start with. You'll probably go from 16 to 14 but you'll actually enjoy driving and accelerating...that's well worth it to me.

[Bowler807]

Sorry I mis read the OP's post.

[LAGA]

About a month ago I did a 3.73 to 4.10 swap in my 07 VMAX. A lot of people say there isn`t that big of a difference but my results say there is. The truck drives and responds a lot better than before. Acceleration is faster and I do not have to use as much gas peddle to get it moving. I do have a heavy foot and want performance in a truck, that is why I went with the VMAX. I am very happy I did the swap. On the plus side, my mpg, on the average has gone up. When I can control my foot. On the highway even through I am turning more rpm I have found that the AFM stays on longer because the engine is not working as hard. So even doing 70 mph the AFM will stay on for minutes at a time. I used to average 12-13 mpg with mostly city driving. Now, I am getting 13-14. I have had a Black Bear tune for 18 months and I had Justin change the ECM for the new gears when he was in town. I do believe that these trucks need taller gears, but GM puts in shorter ones for mpg ratings.

[spikesputnik]

On the highway even through I am turning more rpm I have found that the AFM stays on longer because the engine is not working as hard.

That's what I was thinking/hoping could be the result of a somewhat lower rear end. Or is that all AFM effect? Common sense tells me that an engine laboring against too high a gear ratio will have worse fuel economy than one happy to be spinning 10% faster.

Can anyone else comment on this paradox of fuel efficiency, or is it just a fantasy in my head?

After all, GM did spec those low ratios for a reason....maybe they're good for 55-60 mph or around town, before truck (lack of)aerodynamics take over?

 

LAGA- what did that gear swap cost you, if you don't mind saying?

[LAGA]

The OEM gear and install kit from Ron`s Machine shop online cost about $400. I have a friend who is a GM mechanic, he did the install for $100. He needs some concrete work done which is what I do. The install at a shop runs about anywhere from $400 to $600. You can get cheaper gears like Motive, Yukon, and others, but I went with OEM and they are absolutely quiet. Another thing I have noticed about the swap is that there is way less back lash in these gears than from the factory. There is very little play in the drive train when driving around. And you know that thump you get when sitting at a stop light? It is gone.

[spikesputnik]

Thanks LAGA. Good stuff always seems to come in $500 chunks, or multiples thereof .

Uh, no on the play and thump(yet?). I thought the later was a yoke binding thing anyways?

I don't know how instantaneous the DIC avg mpg thing is after a reset, but this morning I topped off the tank, reset it to 0mpg, went 40 miles at 80 mph 2100 rpm and it said 14.4 mpg getting off the freeway. A couple of hours later did the same stretch back( no hills, very little breeze) after a reset to 0 mpg again, and got 16.3 mpg doing 75mph 1900 rpm. 5 mph = 2mpg diff? Either way it unlocked only once or twice so it was relatively happy either way, just thirsty first thing this morning? Next time I'll do 75 there, 80 back and see.

OR, is that an inaccurate way of testing, ie using DIC vs refills?

[pm26]

I do believe that these trucks need taller gears, but GM puts in shorter ones for mpg ratings.

 

Actually it is the other way around.

 

3.23s are tall gears, 4.10s are low or short gears.

[Starrman]

I believe these motors get better gas milage if kept under 2000rpm. I loose a couple mpg's while cruising at higher speeds/rpms.

 

As for the gear install.....is your truck 2wd or 4wd? Prices around here at local shops (not stealerships) for a single gear swap run about $300 on the low side to about $500 (both include gears and install). Price would be double if you have 4wd....gotta swap the front too.