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Discussion Starter #1
I have a 91 Cheyenne 2500 with a 6.2L, TH400 3spd, 4.56 gears (military surplus 3/4 ton truck).

Originally I intended this truck to be my local mule for hauling, with rare trips over the highway and took to advice to leave well enough alone. However- I have found myself using this truck a little more frequently over the road.

I'm considering swapping the gears to 4.10 and next tire change go to a 265 75 16 or larger. What would you guys recommend? My fuel usage right now sucks horribly (Maybe 10 mpg) and sustained top speed limited to 50-55mph tops.

I don't want to create a truck that will bog down on the mountain roads (it's already very sloooow going up hills with a load of firewood) but I don't want to spin high RPM on the open road either..

Any help / advice would be appreciated. Thanks.
 

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Wow. That is terrible fuel economy. Even with those gears you should be getting better mileage than that. Maybe you have a leaking fuel line or something.

Anyway, I think I would go with 3.73 gears. Without an overdrive you should still be able to climb hills, and that should drop engine speed substantially while cruising. My truck has 3.42 gearing with overdrive and it generally doesn't even come out of overdrive when climbing moderately steep hills at 55. At 45 it will come out of overdrive but it still doesn't slow down much at all.

Good luck!
 

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Most H.D. 3/4 ton trucks came with the THM400 and 4.10 gears. Some had the larger diameter 235-85x16 tires. Most had 245-75x16's. I have one of each. The 245's had more power but rpm's were higher. The cruise speed was about 55-60 before it sounded like it needed to "shift again".

I put a 700R4 (with lots of upgrades) in the Suburban, with the larger tire and it cruises at 70 with room to spare. Gets 20 mpg on the highway. Wouldn't pull a heavy trailer without help. (like leaving it in 3rd gear) These are 6.2 NA trucks so I don't expect much in the power dept., but pulling a 5000# trailer with the truck is no problem.
 

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Sounds like the first thing on your list should be rebuilding your injection pump, low power and poor fuel economy are both signs of a pump needing help. Then you need to increase diameter of tires, try 255/85r16, they will work on stock wheels and are about 1 1/2 inches taller than the stock 235/85/r16 tires your truck came with. Next tire size would be 315/75r16, these are 35 inches tall and will probably need some minor fender trimming on stock rims.
4.56 gears are too low for a 6.2 unless all you do is go very slow, they put the engine out of peak power at highway speeds do to too high rpms. Even with 4.56 gears, you should be able to get about 14 mpg driving 60 mph.
 

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Discussion Starter #5
40,000 miles on the truck.. I was hoping the pump would be o.k. How much is an injection pump rebuild?

So it seems (if all else is running right) 4.10 gears and something around 33" + diameter tires should do the trick?
 

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Injection pumps vary by rebuilder, but I would get your pump rebuilt since the military have some hardened parts inside to allow running of different fuels. Some of the early pumps suffered early failures and while the engines would still run, they ran like crap. Find a good pump rebuilder and have them run your pump on a test stand to check it out. Advance piston bores and some small internal parts are usually the culprits.
 

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I used to run my 6.2L Burb with TH400/4.10 and 255/85R16s. It had great hauling power like this. Not ideal on the highway IMO, but it must have been better than 4.56s and smaller tires :eek:

I re-geared to 3.21 for the highway legs - but it completely killed my hauling ability. In your case, I wouldn't go further than 3.73 and expect to haul.

I am quite surprised to hear of a stock '91 model that uses a TH400. I thought all the 8-lug trucks used a 4L80E by then.
 

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In your case, I wouldn't go further than 3.73 and expect to haul.

I am quite surprised to hear of a stock '91 model that uses a TH400. I thought all the 8-lug trucks used a 4L80E by then.
:exactly: 3.73 is the lowest you should probably go to.

By the way, the TH400 was still being used until 93+ (I think).
 

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Discussion Starter #9
I am quite surprised to hear of a stock '91 model that uses a TH400. I thought all the 8-lug trucks used a 4L80E by then.
This was a mil-spec truck and geared like the older CUCV models.

I talked to my neighbor who works on heavy diesels and trucks. He seems to be of the mindset that 3.73 should be the way to go, but is perplexed at the loss of power under load. He said a 6.2L, even though a little smallish, shouldn't have any problems lugging a load of firewood over the mountains.

He detected a 'miss' when the truck was at partial throttle. We cracked each injector to see if one was faulty, but they all pretty much stumbled out the same. He was really perplexed when the truck idles at cold start great (no smoke) and when it reached operating temperature it ran rough and smokey..

In any event- making sure the engine is running right is priority #1, cause if this truck can't handle a load over the mountains with a properly running engine, I'll doubt I'll switch the 4.56 gears out. That would only make matters worse...
 

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sounds like retarded timing. Try advancing the timing about 1/16 inch, about 2 degrees.
 

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Discussion Starter #11
sounds like retarded timing. Try advancing the timing about 1/16 inch, about 2 degrees.
A couple weeks ago I tried that. I loosened the three bolts and tried to turn it clockwise, but it really didn't seem to want to budge, even with steady pressure from a bar.. I think the housing moved 1/16", but no difference when running.

Not being familiar with this- how much pressure does it normally take to move that thing?

Thanks
 

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3.73 and call it a day
 
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