4L Tranny

[LDMoore]

We are in the process of trying to buy an 08 Z71 Suburban w/ almost 200k miles. It's a lot I know but needing something longer and mainly cheaper than our 11 Escalade as we're working on getting out of debt.

I now realize the 6 speed did not come out til 09 which was highly disappointing. My question is if we go through with this one what can I do to make it more reliable and last awhile?

Also what kind of mileage difference have people noticed between the 4 and the 6?

Any tips would be much appreciated!

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[Dustin Jackson]

We are in the process of trying to buy an 08 Z71 Suburban w/ almost 200k miles. It's a lot I know but needing something longer and mainly cheaper than our 11 Escalade as we're working on getting out of debt.

I now realize the 6 speed did not come out til 09 which was highly disappointing. My question is if we go through with this one what can I do to make it more reliable and last awhile?

Also what kind of mileage difference have people noticed between the 4 and the 6?

Any tips would be much appreciated!

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I just picked up an 08 z71 with 160,000. If you haven't heard of the AFM feature on this platform it is known to have a high failure rate so most people disable it with a Range AFM disabler. That's what I did, other than that the 5.3 and 4l60e should work just fine as long as they have been maintained properly. I have been happy with my z71.

 

[LDMoore]

Oh yes I'm definitely familiar with AFM lol although I was under the impression this year model didn't have AFM. I could be wrong though. If it does I will definitely be disabling it.

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[LDMoore]

Anybody else have tranny advice? Or know what years they implemented AFM?

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[swathdiver]

Anybody else have tranny advice? Or know what years they implemented AFM?

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The Suburban will have AFM, all of the 5.3s and the 6.0 in the 1500s had it. Your trans in the 5.3 likes clean oil and the corvette servo mod. But talk to the guys and search this forum before doing it. @01ssreda4

The 2007-2008s were rated for about 14 city and 20 highway depending on the gearing. Lower gears will yield slightly less highway mileage.

If the engine happens to be the 6.0 L76, it will have the Supermatic 4L70. Oh wait, you said Z71, that was LTZ only if memory serves.

@kbuskill Ken here has an '08 'Burb with about 250K, he can give you better pointers than I too.

 

[kbuskill]

The Suburban will have AFM, all of the 5.3s and the 6.0 in the 1500s had it. Your trans in the 5.3 likes clean oil and the corvette servo mod. But talk to the guys and search this forum before doing it. @01ssreda4

The 2007-2008s were rated for about 14 city and 20 highway depending on the gearing. Lower gears will yield slightly less highway mileage.

If the engine happens to be the 6.0 L76, it will have the Supermatic 4L70. Oh wait, you said Z71, that was LTZ only if memory serves.

@kbuskill Ken here has an '08 'Burb with about 250K, he can give you better pointers than I too.

Not really sure what to say...

Keep the transmission fluid changed and clean and add a bigger transmission cooler.

Same for the oil, clean and changed. I run Mobil 1 extended performance 5w30, 15k mile oil.

I run 13k-15k mile OCIs but I have a dual remote mount oil filter setup with filters that are much larger than stock and an oil cooler. Blackstone labs says I could run the oil longer if I chose to.

As far as mileage is concerned... I have 3:73 gears out back with the 4L60E 2wd with the 5.3l AFM disabled in the custom tune, tuned for 89 octane, and I average about 13.5-14 mpg around town and about 18.5-19 mpg on the interstate running 75 mph loaded down with the wife, 3 kids, and the back loaded down with luggage.

My truck is also lowered 4/6 which may improve MPG. Debatable, but thought I would mention it.

I don't think the 6 speed transmissions make that big of a difference as far as fuel mileage is concerned.

 

[swathdiver]

I don't think the 6 speed transmissions make that big of a difference as far as fuel mileage is concerned.

Perfect write up Ken! And you're spot on about the 6-speed with regards to improved mileage. The biggest change was the addition of VVT to the 4.8 and 5.3s.

 

[mikeyss]

I kinda disagree about the 6 speed not getting much better mileage. Maybe in city driving the MPG isn't much different than having a 4 speed, but on the highway is where the 6 speed shines. Because of the lower 1st gear, you can run a better rear end ratio so the engine isn't revved up at 65mph. My 06 6.0 with the 4L65/4.10 barely squeezes out 16mpg and runs 2500rpm at 75mph, while my 09 6.2/3.42 gets between 18-22mpg and runs at 1800rpm at 75mph.

 

[kbuskill]

I kinda disagree about the 6 speed not getting much better mileage. Maybe in city driving the MPG isn't much different than having a 4 speed, but on the highway is where the 6 speed shines. Because of the lower 1st gear, you can run a better rear end ratio so the engine isn't revved up at 65mph. My 06 6.0 with the 4L65/4.10 barely squeezes out 16mpg and runs 2500rpm at 75mph, while my 09 6.2/3.42 gets between 18-22mpg and runs at 1800rpm at 75mph.

I said I don't think... lol

My truck runs about 2000-2100 rpm at 75 mph with the 3:73s. I also run 285-40-22s which are ever so slightly shorter than the factory 275-55-20s but the speedometer has been recalibrated for them.

 

[mikeyss]

I have to add, it screws with me when I drive both vehicles on the same day. I drive a lot in the city, it's crazy how different the 4 speed and 6 speed are. The 6 speed has better ratios and keeps the engine in it's happy place. The 4 speed makes the engine fall on its face at times, especially with the 4.10 rear. If you are going 30mph and drop the hammer, you will NOT get 1st gear because the engine speed is too high. I really wish I could swap a 6L80 in my 06, but the cost wouldn't be worth it.