New rear end gears -

[ck80]

This is all great feedback, thank you. I tried searching but could not find this type of response. I agree car is not just a bunch of parts slapped together, that's why I'm asking. Ultimately I will upgrade the power, 300ish HP is my goal for a DD. I'd also like some bigger wheels but not going with anything bigger than 18, maybe even 16 or 17. The overdrive trans is for a steeper 1st gear and better highway milage. I might not get there tomorrow when I pick up my car but I can't afford to do all the work at once. So maybe the rear gear is not my next change. Maybe its the wheels/tires. Thanks again.

Here's my order:

Decide on what engine trans you plan to run. Then decide what brakes you need to control it and be happy with where you drive.

Once you know that, then buy your tires and rims. Certain brakes won't fit behind certain rims. Lots of guys prefer thebappearance side of thing before the drivetrain just for affordability purposes and to have different different ride/handling.

Next up I recommend thinking about suspension and exhaust. A heavier engine in a worn out v6 suspension is no good. Likewise, a planned exhaust may be possible to start using before the swap, or at least can be budgeted ahead of time so it's on the shelf for the engine.

Then it's transmission time, because the old stocker often dies behind a new engine.

Knowing what valve body you're running you have a toss up. If you KNOW that your engine will grenade the rear end, you may as well upgrade ahead of the swap because what's the point of dropping tons of cash into something you can't drive or it'll pop? Likewise, no point in installing a brake upgrade when the rear is about to be swapped? If your engine plan is set in place you can run projections on a dyno calculator to have an idea where powerbands roughly lie. Typing in tire size, trans, and different gear ratios will tell you what rpm you will turn at different mph. You can pick which gears you want based on where you want rpm to be for how/where/speed you'll run the motor and your power bands on the motor.

Like the exhaust, think cooling. Maybe get a bigger radiator, maybe electric fans, things like that ahead of time. It's money you'll spend during engine installation anyways, so, do some work ahead, save time on swap weekend.

Then, go for motor last and it'll be a quicker easier install. You've already got rims that will clear bigger brakes if you want them. So on so forth.

Obviously depending what you run and depending what engine you want some steps can be skipped. A 305 car can use the factory exhaust on a 383 engine upgrade. Not the best, but functional to get going. A 350/290hr engine won't kill a 7.5 rear quickly unless you're really abusing stuff, so on so forth. But it's a good order to go through a checklist on.

 

[Calgary Rocket]

I got 4.10 gears in mine behind a gm 2004r with a 450+ HP 500 ft/lbs engine...ready for the 7 1/2" ring gear to grenade lol.
Other than that runs about 2500rpm at 65-70mph (255/45/17 tires). A little "buzzy" and slight driveline vibration but takes off like lightning bolt ....when the tires stop spinning lol 😆

 

[Cuse99]

I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?

 

[ck80]

I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?

$1500 into a 7.5 is too much.

Scrap it or sell it for $150 to someone. Take that $1600 or so and add it to another grand and have a very nice turn key setup that's built so strong you'll never look at it again.

 

[39F]

I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?

That's nuts, parts are like 200-250 for gears, seals, shims,ect. is that with locker and brakes also????? Maybe new tires ???

 

[ck80]

That's nuts, parts are like 200-250 for gears, seals, shims,ect. is that with locker and brakes also????? Maybe new tires ???

Probably not all that, but hopefully at least some lube before getting bent over and paying.

 

[57 Handyman]

CK80, here's some additional thoughts...search around for an 8.5 in rear end assembly. With some luck, you'll be able to find someone selling a unit from a Buick Grand National which is equipped with 3.42 posi gears. I swapped that rear end assembly into my 87 El Camino with a stock 305 hooked up to a 2004r. Rear tires were 16" IROCs with 245x60 rubber. The change in overall pep and giddy-up was great! Next major modification is dumping all of the smog with ECM stuff and coupling that drivetrain to a semi-modified, balanced 350. Can't wait to punch it!!!

 

[83MonteCarloSS]

OK, but why? Why these gears over 3.08 or 3.23? What's the advantage?

Whatever the trans was calibrated for as far as gears and tire size is what I would use, unless you are planning on making valve body or governor modifications. Small differences in ratio or tire size would be fine if your not recalibrating the trans. Changing the ratio is going to result in different shift points. Ideally I want my transmission to shift between 2,500 and 3,000 rpm unless I'm really babying it or romping on it.

 

[ck1984]

I've read thru this thread. The first thing you have to do is actually understand the effect of changing a gear ratio and why specific ratios are chosen to begin with. You said you are going to a 200 4R. I ask why. Because people said that was an upgrade or because you expect a certain result? You car is perfectly capable of being a DD with a TH350 and the 2.73. So the swap to the OD is strictly unnecessary from a DD standpoint. The reason to go the an OD is 2 fold usually. Better acceleration and the ability to retain and improve highway mileage. The addition of the OD slows engine speed at RPM. Which seems great on the surface. But the problem is you now move the engine out of it's optimal power creating RPM range. Basically you are lugging the motor and everytime you hit a small grade the car has to downshift to 3rd (drive which is 1:1) to tackle it. So to fix the problem you have to ADD MORE GEAR. Now you are right back where you started. Its a pickle. What happened to your fuel economy increase? Its actually there because the rear gear is actually picked to provide mechanical advantage. The more gear the bigger mechanical advantage. So now your motor isn't working as hard to do the same amount of work (maintain a speed). It is also easier to get the car moving from a light because.... More mechanical advantage. That's why there is a transmission. Now the OD also has a lower (more advantage) first gear to help with city driving. They are in effect a wide ratio transmission. Which actually sucks. Ask anyone who has driven an old wide ratio. So how do you help make the transition from high first to normal second? More mechanical advantage in the rear. Now you want to add a taller tire. Well that has the effect of decreasing mechanical advantage. The only way to accommodate that is more rear gear. So you need to pick the right gear based on all those things. You have already seen that everyone has an opinion. 3.23, 3.42, get rid of the 7.5 blah blah. Only you know what you are building, what motor you have, how it's built etc. For example my 84 Trans Am has a 700R4 and a 3.23 from the factory. I drove that car everywhere for years. I never felt like it needed more gear. I am moving up to 3.73because I am redoing the motor and expect to be making power higher up the band and I'm running a tall tire. My bone stock GP with a 301 and a 3.35 first T5 is running a 3.23 on P255 60 R15s. It happens to be just right. Both cars retain the 7.5 rear BTW. It is perfect adequate for a DD making modest power.

At a minimum some basic math will tell you what gear to use if you have no idea what RPM range you want to use. The step from 2.73 to 3.23 is 18% so you know that with no other change you will increase your cruise RPM by that much over factory. The 200 4R has a .67 OD ratio. That's a 33% decrease from factory if you made no other change. Add a tall tire and you are idling on the highway. Literally. I've done it. Now the first gear is a 2.74 vs the 2.50 that you have now. So with no other changes you have a 10% increase at the bottom. The point of this lengthy diatribe is this. Pick the gear right for you by determining what and how the car will be used. Building a car isn't slapping a bunch of random parts together. To do it right you have to approach it like they did when it was first designed

I have a 2006 F150 truck for winter and 2011 civic for highway driving .
So my 84 Cutlass is just for DD grocery getter or was a going to the gym car everyday. [until yesterday my province re-instated mask and soon Vax passport so i will be working out at home.]
So now its only a weekly summer grocery getter and maybe occasional roadtrip.
It has Th350 mated to 02 5.3LS but still cant peel 1 tire off the line, due to the likely 2.14 gears in rear.

So im looking for 3.08 or maybe 3.42 rear gears. So i may be looking at a 50% more TQ multiplication and faster speed off the line but also 50% more rpm.
what would you recomend?

 

[olds307 and 403]

I have a 2006 F150 truck for winter and 2011 civic for highway driving .
So my 84 Cutlass is just for DD grocery getter or was a going to the gym car everyday. [until yesterday my province re-instated mask and soon Vax passport so i will be working out at home.]
So now its only a weekly summer grocery getter and maybe occasional roadtrip.
It has Th350 mated to 02 5.3LS but still cant peel 1 tire off the line, due to the likely 2.14 gears in rear.

So im looking for 3.08 or maybe 3.42 rear gears. So i may be looking at a 50% more TQ multiplication and faster speed off the line but also 50% more rpm.
what would you recomend?

Is there an electrical connector near the front on the driver's side above the pan? If it is a TH350C, it will run 300 rpm less on the highway. Also make sure there isn't a stud by the cooler line fittings, that makes it a weaker TH250(C). Here is a good calculator. Just put 1 in place of the .67. If you don't the electrical connector, add 300 rpm.

Reference Manual: Chapter 3: Axles: RPM calculation