3rdgensilverRS

I have a stock 305 TBI. I was wondering what would better suit my 700r4. 373 or 410? Also a best buy at either summit racing or jegs?

JamesC

iTrader: (5)

3.23's or 3.42's.

JamesC

3rdgensilverRS

And this will change my speedometer reading as well so I'll need a fix for that too right?

JamesC

iTrader: (5)

If you're changing ratios, yes. You'll have to swap the drive and/or driven gears to make the speedo read correctly.

JamesC

3rdgensilverRS

Alright thanks! I see a a thread on that right now.

Ron U.S.M.C.

A gear ratio change is the perfect chance to add a L.S.D./Locker to get rid of that one wheel peal.
I went from the 2.73 open differential to a 3.42 with Eaton posi. and it has made a huge difference.

DeltaElite121

3.42's are a perfect compromise.. once you start stepping up to 3.73's or 4.10's you start losing durability because of the ring/pinion tooth count/sizing.. and this is already in a weak rear end.

3rdgensilverRS

So do 3.73 and posi. And change the speedo gears.

paul_huryk

Posi and 3.42 gears. 3.73 or 4.10 will accelerate the car quicker, but you may run out of rpm at the top end of the 1/4. Not to mention your top speed and mileage will go down considerably.

1983Chimaera

How big is the mpg drop on the highway with 3.42s? I also currently have 2.73s with posi in a 95 Z28 rear end, thinking of switching them out since most of my driving is 35-45 mph in town with stoplights. The mileage with 2.73s is great on the rare long highway driving I do, but these gears are killing my wallet in town! I'm willing to sacrifice a little on the highway end if I can make it up in town.

RelentlessRS

I just bought a posi and 3.73 gears up from 2.73. Haven't installed them yet, but i have upgraded from the 305 to a 350 with a mild cam. I'm thinking ill have no problem topping out in the 1/4 and top speed shouldn't become an issue.

Ron U.S.M.C.

This may or may not concern some of us but what I would take into consideration is this stop light to stop light, 35mph to 45 mph mostly around town speeds that allot of use will be using as the cars primary environment. First let me say that the subject concerning driving around town at those speeds in “Drive” as opposed to “Over Drive” is a divided one here on this site .Personally I use “Drive” so my trans. will not be locking and unlocking all the time. I have 3.42s with 27in.diameter rear tires and I am right on the edge concerning RPMs. If you decide to use drive around town with 3.73s and smaller rear tires than your RPMs will be considerably higher. I don't know if I would use Drive around town with 3.73s.If you use Overdrive with the 3.73s around town your going to find that your transmission will be doing that lock/unlock all the time.

1983Chimaera

Ok, let me clarify that I have an override for my lockup so it can be disabled. Now back to my original question, what kind of mpg loss will I see on the highway with either 3.42 or 3.23 gears over 2.73 gears, percentage wise? Will I see a noticeable gain from in-town driving with 3.23s, or should I go to 3.42 gears? Will the in-town gains offset the highway losses? It is used primarily for in-town transportation, not as a street/strip car, so I'm looking to maximize my average mpg, not lower my 1/4 ET.

camaromike222

I'm running a 305 TBI with a 700r4 and I did do a 3.73 and Zexel Torsen swap. The pulling felt AMAZING! It really woke up the 305. However, at 70 mph I was turning about 2400-2500 rpm if I remember correctly. Average MPG was maybe 16-17 mpg with mainly freeway use. I made the mistake of not changing the speedo gears, so it read faster than I was really going. And when it read I was doing 115 mph, it hit the top and the governor kicked in and slowed the car down. I'll say this much: They were FUN! downshifted into 3rd and the speed climbed quickly. I do think a 3.42 will suit you best. You should see less rpm and slightly better mpg. You won't be missing much from the extra .31 torque multiplication.

1983Chimaera

Right now I average about 16-18 mpg city, and 26-27 highway with my TPI 333, 700R4 and 2.73 posi rear. I've leaned my idle and cruise out as much as possible, and pretty much have my AFR table tuned perfectly. I have a bypass circuit where I can disable my converter lockup, so that isn't an issue in city driving. I want to get the city side above 21mpg, without sacrificing too much on the highway side. More acceleration would be fun, but I'm more concerned about fuel economy. 2.73 gears are great on the highway, but just too much work for the engine in stop and go driving to be efficient.

3rdgensilverRS

I think you have me hooked here. I'm doing it haha!

Ron U.S.M.C.

This is for the "OP",

If you decide to build your differential from scratch/new parts than this list of what I have done to my 10 bolt may give you an idea of what can be involved.Some of these parts are necessary for the build and some can be changed to match your personal preference as in brand and style L.S.D./Locker.,etc.

1. 3.42 ring and pinion =$ 200.00
2. master bearing kit = $ 90.00
3. axle bearings and seals = $ 30.00
4. 28 spline Eaton posi. Unit = $ 500.00 ( highly recommended )
5. 28 spline axles = $250.00
6. installation/labor = $ 300.00
7. and if you want ,a LPW Ultimate support cover = $ 160.00 ( also highly recommended )
   Thats $1,530.00 counting the cover.

Here's a link to Eaton = http://www.eaton.com/EatonCom/Produc...iLSD/index.htm and here's a link to the LPW cover = http://www.jegs.com/i/L.P.W./619/301-7.5G/10002/-1

DeltaElite121

^ with Ron; I also have a LPW cover. If you order directly from LPW it's much faster, and it's a few dollars cheaper. Make sure you get the brace for it as well.

1983Chimaera

Kinda curious why you're advising a junior member to spend over $1,500.00 on a rear end to put behind a stock TBI 305...

DeltaElite121

I don't think he was ADVISING it moreso than showing what it would cost in the event you start pumping money into it.

Personally, I went with a LPW cover and a brace.. if I blow the rear end I'll just sell the cover/brace and move up to a 9". That's what I'd do.

Phenom-1

I'd say 3.23s for an auto and 3.42 up to 3.73 for a m5 and m6

Ron U.S.M.C.

Maybe I should have made it clear by stating that there is a cheaper ways of doing it first. I can see were it could be interpreted in that manner but was not my intention,....so I did fail to mention things like this.He could find a used rear that fits his needs or he could do it with a 3.42 gear set ($200.00)and stay with the 26 spline axles , he will need a master bearing kit and axles bearings and seals ($120.00)IMO there is no reason to do all this work with out installing at least a locker(26 spline),lets say its ($300.00 ) and approximately $300.00 for installation/labor.(That set up or a variation of it). He could possibly get out the door for $920.00 not including tax..I'm not a mechanic and am still learning as I go along with my build... I know passing on the opportunity to upgrade to 28 spline axles and carrier is a option but I chose to upgrade.Buy the way this is my first project car and I bought it for $500.00 and pulled it out of some guys back yard. It took me almost 2 years to save the money for that rear,and another 3 for the paint.I understand that's a long time to wait but that's just the way I am.

paul_huryk

Probably lose 2 or 3 mpg on the highway going from 2.73 to 3.42.

If you decide to redo your rear, do not go upgrade to a 28 spline posi or even new axles (unless yours are toast). While it can happen, broken axles make up only a small percentage of 10 bolt failures - it is usually the ring gear that rips up...

DeltaElite121

which buying the girdle brace (for the LPW girdle) will help prevent since it puts a pre-load on the teeth to help that problem. I agree with you.

camaromike222

Given that it's a stock 305, find a used set of gears or a complete 4th gen rear end. Then spend a little cash on the cover/girdle. You'll get rear disc brakes, a nice 3.42 gear ratio and a little bit of added strength from the girdle. Summit sells a nice one. I have the Moser one. These thing are all the same as long as they have the main cap support. Nice fill and drain plugs. You don't need to spend too much. If you're brave enough, you can maybe do the swap yourself. It's very involved, but if you have the right tools, it can be done in a few hours. If you by chance go down that route, PM me and I'll will be more than happy to run you through it and continue helping. Either way, you'll have fun with your setup

DeltaElite121

LPW is the only one that offers the pre-load brace kit.. which has apparently helped several claims that have tried it in higher HP applications. Not being a stickler; just throwing that out there.

camaromike222

^^^That's true. I suppose it would be a good idea to buy that girdle and later get the braces if power lvl's increase. I forgot about that lol. I think Jegs sells the brace kit with their girdle not sure how the fitment is on our 10 bolts, though. Anything to keep the ring and pinion lined up will prolong the the life of these things. Spend as little as you have to now, then just slowly upgrade.

yaj15

iTrader: (3)

You have a very nice car. I followed your thread when you rebuilt your rear end. I'm in the process of doing the same thing with my borg warner 9bolt.

Yes a good amount of money was spent. With this kind of job, however it's not the time to cut corners. You want this job to be done right the first time and not have to go through it again.

For a stock mostly stock 305 or 350 powered thirdgen 10bolt or borg warner 9bolt is all that you will need. The way this 10bolt was rebuilt, when more power mods are added the rear end will be up to the task. Sometimes its strange. Someone will put a lot of money into the engine and transmission then cut corners when it comes to the rear end or not pay much attention to it at all. The rear end is also critical in getting your engines power to the pavement. You can have a car with 500hp racing a stock 1960's VW Beatle and lose to it if your rear end is not up to the task at hand.

Until you really need to go up to an aftermarket 9 inch or 12 bolt rear, I would stick with running a 10bolt or borg warner 9bolt rear. Our cars came from the factory with these rear ends so (depending on your car) there should be no special mods or brakets needed to hook up the sway bars, brake lines, etc. to get it to fit into the car. Also remember that the bigger the rear end is, the more power it will take to turn it. Running a 10bolt will be more efficient due to less weight and less rotating mass. You can also run factory style brakes. This is not always the case with aftermarket rear ends.

After you get your power up high enough to need an aftermarket rear end, the power loss through the larger rear end will just have to be accepted as part of the program. Like others have said one of the weak links in the 10bolt is the small diamter ring gear and the fact that the case distorts under high load. Welding the axle tubes and running a differential cover are realtively cheap measures to add strength to a 10bolt rear.

paul_huryk

I never heard of 10 bolts being saved by a stud girdle - know plenty of pleople who put them on and still ripped teeth off...