Well, got my pickup back today after having the gears changed from stock 3.55 to 4.11. I went with Yukon gears, and an Auburn limited slip in the rear. The front is the new Yukon thick gear, which doesn't require a new carrier.

Definetly a noticable boost in ability to accelerate, though I think I need to take it to a dealer to have the PCM adjusted for shiftpoints (now that the rpm's are higher, feels like it is shifting at the wrong time). Wish I had a tac so I could say for sure just how much the rpm's have changed, but it sounds noticable (there goes my gas mileage ). I am able to drive in overdrive now and actually accelerate without it downshifting right away. Before, it would lug down easily and would shift out of overdrive.

One noticable downside that I will be keeping a very close eye on. My transmission seems to be running about 10-15 degrees hotter then it was before. Used to average about 165 deg, but would go as high as 180. Today, it averaged 180, and got to 195 as the high. Guess this is due to the rpm increase. Tomorrow, I will test it out on my usual drive to work. That should be the best test of how much things have changed. May need to install a bigger transmission cooler.

 

Today on my drive to work, I definetly noticed a difference. I was able to stay in overdrive over most of the hills that used to cause the transmission to shift out of overdrive. Now, when I am driving 65 in overdrive, it feels like what it used to feel like if I would drive 65 with the overdrive off. That is more of an RPM difference then I expected. However, I am still running my 265/75/16 which is a 31.6 diameter tire. The reason I did the gears was to be able to run a larger tire without bogging the engine. I am thinking that having bigger tires (going to be running around a 33" tire) will make it feel better, and not quite rev so high at highway speeds. Is that true, or are these the rpm's I will be at from now on? If so, could the dealer adjust the shift points or some other PCM setting to help compensate?

Otherwise, loving the gears so far. Probably be a different tune when I go in for a fillup.

 

how are you breaking them in? I had to stop every 20~30 minutes or so to let them cool down up until I had 500 miles on them. then had to go swap out the diff fluids.

big difference huh?

 

oh yeah, you definately need the tires now, go two West Coast Diffs and punch in your info to find out what the rpm change was and what it will be after the tire change. keep in mind that when you go to the bigger tire, your rotational mass will be greater and you'll have lower throttle response compared to the 265's...

 

Ya, that break in you did sounds about like what I am doing. They basically said that normal driving around town to go to the store, etc... accomplishes the break in nicely. Basically drive for 20-30 minutes, then let things cool down, then again. They said do that for about 500 miles, but didn't mention needing to change fluid again. I'll call and ask them about that.

Using the gear chart, here is some detail:
At 65 mph:

With the tire I have and stock gear, the way I was before the gear change (265/75/16, 3.55)
RPM = 1717

Here is how I am now (265/75/16, 4.11)
RPM = 1988

Here is how it will be after new tires (285/70/17 or 33"x12.50R17, 4.11)
RPM = 1904

That is only 125 difference from the way my pickup came from the factory (245/75/16, 3.55, RPM 1779).

That should feel a lot better.

Reid

 

you might be looking at the wrong part of the chart... with an auto, and 285's you should be around 2k in od... have another look. 2k is what I get when I do my truck with 285's and 4.11's at 70 mph

 

Ya, I just edited my post to reflect that. I was looking at the wrong column.

 

yup, now it looks right... cool!

 

So you think that would be ok for normal driving? I know it will be great when I am in the mountains or towing.

 

yes, it works for me and I have the Sierra's to go over, Sacramento is at 25 ft elevation, summits are up above 7k.

I would swap the diff fluids regardless of what the shop says. new gears or anything internal to a motor should have the fluids swapped out after about 500 miles... any internal metal that results from the gears seating will want to be flushed out.