12 bolt rear end question

[maxreps]

Mine has richmond 4.10's and an Eaton posi unit in it but I want to swap gears and go to a 3.08-3.2X(future700R4 hopes). I will have to get an entirely new Eaton unit won't I, since they have one for 4 series gears and one for 3 series gears?

That really stinks if I do because the Eaton is very nice and basically new too. It was put in with the gears about 6 months ago, by the prev owner. Maybe I could sell/trade one or both items or something.

[cochino12]

yep I think you will need to change it all out. No big deal its jut $$$ right? ha ha

[maxreps]

Yeah only money, not! the gears and eaton are $660 from Summit, I hate to "waste" money or stuff when it is perfectly fine... 700R4 would be nice though.

[J_PEPIN87]

If you decide to sell the carrier with the gears I might be interested depending on the price

[piecesparts]

I would suggest a slight rethink on a 700R4 with an output number like .075 and a 3.08 geared rear end. I had a combo like that in my son's truck and the gas mileage really stunk bad. We went back to a 3.73 geared rear end and got back 2 MPG and some change. The RPMs were so low that in the hills of Kansas, with the 3.08 gears that we were working the truck's engine a lot to make a decent trip down the highway. We were also running a smaller tire diameter than most trucks and if you go taller than the 26" tires that we had, the RPMs will be lower, yet. I would think that a 3.42 gearing or the 3.73 will give you what you would want, but you neeed to decide what RPM you want to run at and what kind of driving you will be doing.

I am running a 385 stroker motor, with a 700 R4, 3.73 posi Eaton 12 bolt and 28" tall tires on the truck in my avatar. I am getting a good 15 MPG with an approximate 400 HP engine and a good solid acceleration. At 75 MPH on the highway, I am running around 2500 RPM and I could drop that another 150 RPM with a taller tire install. I would hope that you can do better with your plan, but I have two trucks that are relatively the same in size and setup and they are getting about the same mileage. The Son's truck suffered bad with the 3.08 gears and the OD tranny, so we copied my stroker setup.

[maxreps]

Thanks for the info, I hadn't done a great deal of thinking about cruise rpm with the 700R4's .70 final gear. I was concentrating on the lower first gear and that it'd make the 4.10's seem like 4.90-5.00's which is very tall. If I drop back to 3.73 I think I can still use my current Eaton... which would save alot of trouble selling a good one to buy new one!
My current tires are 29"OD and I kinda like them with the rallies for that old classic look. The 3.73's will have an effective final drive ratio of 2.59 when used with the 700r4 which is lower than the original 3.40 and th350 that the truck came with.. not counting that mine has 29" and it came with 28" originally.
Even the 4.10's will be 2.87 final with the 700r4.. ugh decisions decisions!! Did you guys use any special 700's or just used/rebuilt ones. Some folks have went with trannies from Bowtie OD in Ca but I am wondering if a regualr rebuilt one would be ok if the motor is less than 400hp. Not sure what mine is but I'd guess about 300hp

[westbrookw]

Richmond makes a 4.10 gear that runs on a 3 series carrier. Get sa much info off of the carrier and call 866-589-GEAR. It is a website that I called when I was putting mine together. They were very helpful. The website is www.rearendparts.com. I think the guy I talked to was at extension 118 or 112. Great guy, lots of info.

[piecesparts]

The common breakover point between series 3 and series 4 carriers is around the 3.42 range. For everyday operation the 3.42 is very common, however the acceleration of a 3.73 gear is really nice and you can drive it comfrotably. Mine has done several Hot Rod Power cruises, with no problems.

My Daily driver is a 91 Silverado with a 3.42 gear and a 5.7 engine and 700R4 setup. The TBI is getting me around 20 MPG daily. However that is with a lot lighter truck than the mid 80s were.

[fast75]

Originally Posted by piecesparts

I would suggest a slight rethink on a 700R4 with an output number like .075 and a 3.08 geared rear end. I had a combo like that in my son's truck and the gas mileage really stunk bad. We went back to a 3.73 geared rear end and got back 2 MPG and some change. The RPMs were so low that in the hills of Kansas, with the 3.08 gears that we were working the truck's engine a lot to make a decent trip down the highway. We were also running a smaller tire diameter than most trucks and if you go taller than the 26" tires that we had, the RPMs will be lower, yet. I would think that a 3.42 gearing or the 3.73 will give you what you would want, but you neeed to decide what RPM you want to run at and what kind of driving you will be doing.

I am running a 385 stroker motor, with a 700 R4, 3.73 posi Eaton 12 bolt and 28" tall tires on the truck in my avatar. I am getting a good 15 MPG with an approximate 400 HP engine and a good solid acceleration. At 75 MPH on the highway, I am running around 2500 RPM and I could drop that another 150 RPM with a taller tire install. I would hope that you can do better with your plan, but I have two trucks that are relatively the same in size and setup and they are getting about the same mileage. The Son's truck suffered bad with the 3.08 gears and the OD tranny, so we copied my stroker setup.

kansas has hills?????????

[maxreps]

Originally Posted by westbrookw

Richmond makes a 4.10 gear that runs on a 3 series carrier. Get sa much info off of the carrier and call 866-589-GEAR. It is a website that I called when I was putting mine together. They were very helpful. The website is www.rearendparts.com. I think the guy I talked to was at extension 118 or 112. Great guy, lots of info.

Thanks! I thought that 3.73 was the highest made for the 3 series.. I'll check.

[piecesparts]

Originally Posted by fast75

kansas has hills?????????

Well, they are not like mountains or the Arkansas foothills, but where I live there is this area known as the "Flinthills". The hills are close to each other and at enough of a grade to make the vehicle try to control speed down one side up and work like hell up the other. If you are towing it is enough to make the truck work it's tail off to keep the load moving. If you are running a cruise, the control is trying to kick down the tranny into "Passing Gear" to keep RPMs up. Going to the 3.73 gears from my original 3.42 was a noticeable change in that respect. My son's truck would not even pull any one of those hills without kicking down---with those dang 3.08 gears. Now that rear end lays out behind the garage drawing dust---waiting for someone to haul it off.


Kansas is only flat where we let the Texans drive through, that way they don't impede the traffic flow.

[grinch4om]

3.42, 700 R4, 29" tire...cruises 1800 @60. crap mileage, great acceleration and driveability...mileage may be due to the 454...LOL

[maxreps]

HA!! I am betting so because 1800 cruise seems decent.

[fast75]

Originally Posted by piecesparts

Well, they are not like mountains or the Arkansas foothills, but where I live there is this area known as the "Flinthills". The hills are close to each other and at enough of a grade to make the vehicle try to control speed down one side up and work like hell up the other. If you are towing it is enough to make the truck work it's tail off to keep the load moving. If you are running a cruise, the control is trying to kick down the tranny into "Passing Gear" to keep RPMs up. Going to the 3.73 gears from my original 3.42 was a noticeable change in that respect. My son's truck would not even pull any one of those hills without kicking down---with those dang 3.08 gears. Now that rear end lays out behind the garage drawing dust---waiting for someone to haul it off.


Kansas is only flat where we let the Texans drive through, that way they don't impede the traffic flow.

too funny, was just giving you the blues buddy. Kansas really is a beautiful state, i used to go thru there alot when i was driving long haul.

[LONGHAIR]

Unless you do a lot of highway driving, I would stick with the 4.10s. In-town driving and any kind of towing or hauling will really appreciate it.

[piecesparts]

Originally Posted by fast75

too funny, was just giving you the blues buddy. Kansas really is a beautiful state, i used to go thru there alot when i was driving long haul.

We definitely have some lonesome spots to stop in. Apparently You Texans have something North of us that you all go to see. Just about every fourth car on I-35 corridor is a Texas plate. Good to have you, just stop and see us sometime.