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Old 03-25-2015, 04:09 PM   #14 (permalink)
oil pan 4
EV convert
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Join Date: Jul 2011
Location: NewMexico (USA)
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Sub - '84 Chevy Diesel Suburban C10
90 day: 19.5 mpg (US)

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Bug - '01 VW Beetle GLSturbo
90 day: 26.43 mpg (US)

Sub2500 - '86 GMC Suburban C2500
90 day: 11.95 mpg (US)

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Originally Posted by aardvarcus View Post
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 ton diesel suburban project, so I will be watching with interest. Oh, and we want pictures!
Oh its 2wd. Its a C2500 (GMC). I found that with 4wd you spend more time working on it or around than actually using it.
Plus it seems like no one wants old 2wd suburbans so they are cheap.
It appears the only way to get a 2wd 3/4 ton suburban with full float axle, towing package with trailer brakes, 40 gallon fuel tank, class 4 hitch, heavy duty axle was to get one that had a 454 gas engine and TH400. Which is fine because I already have the engine I want.

My specification was to find a 2wd. Had to be 2wd, I did not want 4x4 at all.
With what I am going to tow with it I really should have a 3/4 ton and full float axle, I already am pushing the limit of what standard 15 inch tires on 5 lug wheels can carry. I found that late 90s and 2000s jeeps use 16 inch 5 on 5 wheels but then the 16 inch load range D tires exceed what the rest of the vehicle should be carrying.

I will be adjusting the rubber gearing. The half ton had 31 inch load range C tires. The 3/4 ton has 32 inch tall 10 ply tires on 16 inch wheels.

The 6 speed allison transmissions I can find are around $2200, that doesn't include freight shipping. Then the standalone controllers appear to be at least another $1500. Then I would need a custom drive shaft. That trans with a controller shipped to me I am looking at about $4000, plus whatever a custom drive shaft costs.
I can get the TH700R4 rebuilt and bullet proofed with a new torque converter for about $1400 to $1500.
If I could pick up a used 6 speed Allison locally for cheap, yeah I might do it.
The 5 speed Allison has the same top gear ratio as the 700R4, so that wont help fuel economy.

I don't want to do an NV4500 swap because my engines internals were balanced by a machine shop in Virginia beach. Swapping from a flex plate to fly wheel would mess with my engine balance.
1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.

Last edited by oil pan 4; 03-25-2015 at 04:55 PM..
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