Gearing options for a 3/4 ton

[Camarrow]

I have read the posting about the best gearing w/ a 700 but what if you have a 400. I am looking at a 86 chevy K 20. It has a th400 and 4.11s. The truck is a pretty good deal and clean. What is the best option for getting better mileage. I could swap the 400 for a 700 or get a GV overdrive. I could also swap to 3.42s (highest gear for a 14 bolt 9.5"). If I swap trans I loose the toughness of the 400 (plus at least costing 2000+ for a trans/rebuild and a transfer case or an adapter). A GV OD swap is about the same price but you only get a .78 OD and have to shorten the drive shaft. A 3.42 swap is a little cheaper but still has high revs at 65 and my lose some lowend grunt. I know if I want good mileage I shouldn't get a 3/4 ton but I do help on the farm and most of my 1/2 tons have been to light. I plan on adding 33" tires to lower the rpms some. How bad is a 400/4.11 on mileage at 65?

[NH2112]

Personally, the absolute tallest I'd go in a 2500 is 3.73 but other than the noise level dropping a little bit I don't think you'll see much in the way of better mileage. Then, if you do go to 33s you'll probably find the 4.10s would have been better. It'll probably take an awful long time for your fuel mileage increase to offset the cost of the gear swap.

[wthif]

I personally like my 4.10s in my 3/4ton and TH400 31" tires (stock). I've seen 20mpg but average right now is 10-14mpg do to a lead foot (floored most of the time, while accelerating ) and injection pump, injectors, timing chain need to replaced. Also I live in some hills and its rare that I get on flat lately and maintaining 55-65mph in the hills isn't easy on the millage. Just don't plane on going faster than 80mph. I usually don't go faster than 65mph. I imagine once I replace the injection pump, injectors, timing chain and Have it timed I'll get a consistent 15 mpg. I also would never consider higher gears for my truck, the low end grunt is great :D

[Diesel Dan]

Due too the tall first gear in a th400 if you are planning on towing then use the 4.10 gears. Then get a Gear Vendors OD. Funds permiting.

I like the 3.42s in my '93 but I have a manual trans to give me good gears for taking off. I put a 3.36 gear into a '89 crewcab with TBI 454/TH400. Nice for the highway but really hurt the lowend when loaded.

I also had a '88 crewcab, 6.2 N/A, TH400 and 4.10s and installed a GV unit. Mileage went up maybe 2 mpg, around 18 mpg, but what really made it worth it was the comfort crusing speed increased. Before it liked about 62mph, after the GV 70-75 was good. Before the GV 77mph top speed, after-90+.

Recently I borrowed the truck to pull a 5,000lb unimog on my car trailer. This was the first long haul trip I had used the GV pulling a load. It's the cat's meow for towing :D .

[Camarrow]

I guess the heaviest thing I pull for a distance is a car trailer with a BBC camaro on it. Other wise I may pull some heavy stuff around the farm but a low speeds (under 45 mph). The biggest thing is I drive 60 miles plus a day to get to my real job before I go to the farm. I mostly only go 60-65. At 2 mpg it would take a long time to pay off a GV OD in fuel savings. That is why I contimplated a gear change cause since I have the front gears (I think, 10 bolt 8.5 right) I could do the gear change for under 1000 and most likely under 500. 3.42s may be high but then I get to keep the bullet proof th400 and possibly improve mileage. I wonder how big a gain though. If I could get 18 I think I would s--t myself.

[84 Convert]

Prior to blowing the head gaskets out of it, I was seeing 13-16mpg in my CC dually 4x4 with an SM465 and 4.10's. Seemed that the load didn't matter, but the fuel did. I may get better after the gaskets are fixed though I doubt I was losing much compression because it still started fine. I tend to agree on not upping your gearing if you tow much. My dad had a 2500 with3.42's and a th700... had to use my '74 Dodge 1/2T with a 360 to get a Bobcat up our hill...the 6.2 wouldn't do it. On the other had, my CC dually will with 4.10's. I run 235/85R16 tires which are about equal to 33" for height. Oh, yeah the 2500 would get 25mpg if babied!

Hope this helps.

Gregg

[Craig M]

On the GearVendors unit the nice thing besides the top end speed is using the OD in 1st and 2nd gear when towing. A six speed automatic keeps speed up on the big hills much better than a 3 speed automatic. Still not as good as a 15 speed in a big rig but a nice improvement for the automatic.

[Camarrow]

I guess the thing is what would the mileage increase compared to the cost of the Mod be. I mostly drive 60 miles to work everyday. After work I go play farmer. I mostly only haul with my truck. I do pull a car to the drags but will most likly use are new avalanche cause of the interior room and the towing distance (3 hours one way). Other wise I do pull some 10000 plus trailers but at low speed less than 15 miles. We also pull these trailers with our 80 K-10 with a 350/350 and 2.73. I know this is wrong but it is done so I think a 3/4 with a diesel will pull the same or better with 3.42s and a th400.

[ZZ]

I swapped rear-ends in a 1985 3/4 Chevy 6.2 stick shift I had a couple of years ago. I went from 4.11 to 3.73. It was hardly a noticable change with noise and the fuel mileage increased only 2 mpg at best. It was a wasted $180 to me.