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Old 03-24-2015, 06:54 AM   #3 (permalink)
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Join Date: Apr 2012
Location: Evensville, TN
Posts: 651

Deep Blue - '94 GMC Suburban K2500 SLE
90 day: 23.75 mpg (US)

Griffin (T4R) - '99 Toyota 4Runner SR5
90 day: 25.43 mpg (US)
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Oil Pan,

What a project! What year of suburban, and 2wd/4wd?

In the short term, some 235/85R16 tires would provide some good “rubber gearing” to go along with your 3.42. The only other option I know would be an axle swap for a D60/D61 rear axle with a 3.07:1, but that may be more work than you want to want to for just a 11% reduction.

For the long term solution, are you set on keeping the automatic transmission? I had headache after headache on my 4l60 (which is basically just a 700R4) behind my 350 in my 1991 Chevy K2500, until I finally swapped it over to a NV4500 stick shift. That was without a doubt the single best fuel saving mod I have ever done on a vehicle.

If you want to keep it an auto, you could always try to find an Allison 1000 from a 2001-2007 Chevy 2500HD/3500 with either the 8.1L Vortec or 6.6 Duramax. Those are 5 or 6 speed (6 speed= double overdrive) depending on the year and built like a tank. I would think you could get one out of a wrecked 8.1 gasser reasonably cheap. They make stand alone TCMs to control them, as they are popular for cummins swaps.

I have dreams of a future ¾ ton diesel suburban project, so I will be watching with interest. Oh, and we want pictures!
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