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School me on what i want on gear ratio. 83 factory diesel

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sheepdog_tx

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So I have a civie 1983 6.2L diesel (not a cucv if that matters) 4x4. I know nothing about gears or torc/horsepower but trying to learn. Not sure what I got for gears but I want a decent mix of occasional rock crawling power and street ride. Heard the gears on this V8 are turned way down, not to expect much acceration. IF i was going to get gears changed in whats recommended front and/or back?
 
As mentioned above the tire size and transmission play a big factor in what gear ratio I would recommend. In general the same gears as you would use in gas trucks for the particular combination is a good starting point, with maybe one step higher. For example if 4.56 for a gas engine, than 4.10 for the diesel.

My '90 K5 with the 6.2 diesel came from the factory with 3.73 gears and the TH700r4 overdrive trans. It had 32" tires when I bought it, and I later upgraded to 33's. These axle gears worked pretty good. Later on I switched to 4.10's and have ran 33's, 35's, 38's, and 39.5's on the same gears. Almost no seat-of-the-pants difference between the 3.73 and 4.10 but it worked great for the 33 and 35's on the highway and trail. After going to 38's the overdrive was pretty useless but I didn't care as it was becoming a mainly off-road toy. I drove it on some highway trips before getting a trailer and it did okay, and was fine in low range on the trails. It was a trailered toy by the time the 39.5's came along but still does okay on the road and 4 low is good enough. Wouldn't mind having lower gears but it's low on the list.
 
6.2Blazer said:
As mentioned above the tire size and transmission play a big factor in what gear ratio I would recommend. In general the same gears as you would use in gas trucks for the particular combination is a good starting point, with maybe one step higher. For example if 4.56 for a gas engine, than 4.10 for the diesel.

My '90 K5 with the 6.2 diesel came from the factory with 3.73 gears and the TH700r4 overdrive trans. It had 32" tires when I bought it, and I later upgraded to 33's. These axle gears worked pretty good. Later on I switched to 4.10's and have ran 33's, 35's, 38's, and 39.5's on the same gears. Almost no seat-of-the-pants difference between the 3.73 and 4.10 but it worked great for the 33 and 35's on the highway and trail. After going to 38's the overdrive was pretty useless but I didn't care as it was becoming a mainly off-road toy. I drove it on some highway trips before getting a trailer and it did okay, and was fine in low range on the trails. It was a trailered toy by the time the 39.5's came along but still does okay on the road and 4 low is good enough. Wouldn't mind having lower gears but it's low on the list.
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Not sure on trans but I plan on doing 35s on tires with a 4 to 6 on lift, no bigger than that for sure. I want it to be an occasional daily driver backup in good weather (I want to go doorless/topless) but offload capable to at least a 4 tier trail. So is the standard gears going to be ok or do I need to adjust anything?
 
I have 4.56 gears with a 6.2 a 700 and 35s. It is a 65 to 70 mph rig. It really starts dropping mileage. I think 4.10s would would good for a rig that does alot of highway but I like my 4.56s.

Also I have probably put well over 3k highway miles on it in the last couple months. Its not an occasional driver it actually gets driven dailey
 
I think the 6.2L CUCVs came with 30" tires, 3.08 gears, and a TH400 transmission (which didn't have OD). If you have a TH700R4, you'll want something numerically higher than 3.08.
 
I have a 6.2L TD and run 4.10 gears with 35" (315/75) tires and a Th400 transmission. It pulls good, but really winds up on the highway speeds (70 mph). Works OK for me as it is my tow rig and I am usually going 55-60.
 
You would have to pull the diff cover on one axle and read the numbers on the ring gear to know for sure what gears you have now. But 4.10 and 35s is a pretty good combo for what you have.
 
Mikey von said:
I have a 6.2L TD and run 4.10 gears with 35" (315/75) tires and a Th400 transmission. It pulls good, but really winds up on the highway speeds (70 mph). Works OK for me as it is my tow rig and I am usually going 55-60.
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curious what you gas millage is like, assuming radials not mudders?
 
An '83 civilian model should have a TH700r4 from the factory and wouldn't be surprised if it had 3.08 gears, but no way to know for certain. The VIN number should tell you what axle gears came from the factory, but again you are talking about a 30 year old truck with an unknown history so who knows what might have been changed.

I would lean towards either 4.10 or 4.56 gears with 35" tires.....if a lot of 60+mph highway than go 4.10, mostly around town and backroads go 4.56. Gear the axles to what makes it drive the best on the road. In my experience small steps in axle gears don't make or break a rigs performance off-road.........never heard somebody say "if I had 4.56 gears I would have made that hillclimb, but I only have 4.10's and couldn't make it". My K5 consistently got high teens to low 20's MPG with 3.73 gears and 32" mud tires, and a heavy right foot. Lost a couple MPG's with the lift and 33's, and no clue what it got with 35's or larger as I didn't pay attention anymore.

Speaking of transmissions, the TH700R4 is a much better trans behind a 6.2 in regards to gearing. The first gear is much lower as compared to a TH400 which gives better acceleration and off-road crawling ability, plus it has OD. The military K5's feel doggy because of the high 2.48 first gear of the TH400 combined with crappy 3.08 axle gears. That only gives you a 7.6:1 take off gear. On the other hand a TH700r4 has a 3.06 first gear and you can run 4.56 axle gears to give a 14:1 take off gear yet still cruise 70 mph at the same rpm as the military version.
 
What about crawling under and check to see if the tags are still on the covers and or a build code stamped on an axle tube?
 
Csm Davis said:
What about crawling under and check to see if the tags are still on the covers and or a build code stamped on an axle tube?
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I hear people talk about tags on the diff cover listing the gear ratio all of the time, yet have never actually seen a Chevy truck with one. I bought my 2500HD new and my K5 when it was only 4 years old with 49k on the clock (never looked like the diff cover had been touched) and neither had tags. That doesn't count the dozens, if not hundreds, of trucks and axles I've looked at over the years......though I do understand that if the diff cover has ever been removed there is a decent chance that tag is gone.
 
6.2Blazer said:
I hear people talk about tags on the diff cover listing the gear ratio all of the time, yet have never actually seen a Chevy truck with one. I bought my 2500HD new and my K5 when it was only 4 years old with 49k on the clock (never looked like the diff cover had been touched) and neither had tags. That doesn't count the dozens, if not hundreds, of trucks and axles I've looked at over the years......though I do understand that if the diff cover has ever been removed there is a decent chance that tag is gone.
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Would the 6 digit production code tell me? VIN card doesnt show (https://service.gm.com/dealerworld/vincards/pdf/vincard83.pdf). My VIN is 1G8EK18C5DFxxxxxx, a 6.2L diesel. Also trying to tell if I got a 3 speed or 4 speed trans and if 400 or 400r.
 
I've found tags on front diffs on my '71 and '72 trucks,(K-5 and K-10),but I had to pull the cover to determine the ratio on all the ones newer,or "guess" by jacking it up and counting the number of turns the driveshaft made VS the tire and dividing the results..the most common ratios are 3:08,3:73,4:10...but there are others ...I guess either the tags were lost or never put there to begin with on the 73 up trucks I had..

I have found the axle code on the axle tube and had a GM dealer look it up for me to determine which ratio the truck had--but I had to wire brush the entire tube(s) on both sides to find the code,a three letter code like RAQ,and it was almost impossible to read,it was not stamped very deep and rust almost "erased" it...if you have the options list from the glove box or inner front fender,you can tell by the codes which axle you have that way..

In the end,taking off 10,12,or 14 bolts is faster,and the only sure way to tell just what ratio you have...I bought one truck that had a 3:73 rear axle and a 3:08 front axle,so its best to check both!--I had to buy another 3"73 front axle,or else never put it in 4wd--..
 
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