New rear end gears -

[ck80]

I'm going to be *that guy*....

If you're not going to be installing the gears yourself (and no blame/judgment there) then you're looking at a decent chunk of change in that process.

Depending on your long term goals, if you add the cost of new gears, carrier, labor, etc... I don't think it would be THAT much more in the grand scheme of things to put a stronger/better rear end under there.

That way when you decide to go bigger motor - notice I said when, not if - that upgrade was already done and you're not spending money twice.

If you just want the other ratio, find a monte ss being parted or upgraded. 3.73 with posi complete drum to drum shouldn't be more than 3 or 4 hundred complete and ready to drop in. And THAT is a repair I think you or anyone could handle themselves.

But to buy a carrier, buy gears, pay labor to pull apart the existing rear and upgrade everything... assuming a shop will use your used rear parts from a liability standpoint, I just think you're money ahead thinking about the big picture. And best case scenario you just spent a pile of money .... on a 7.5

 

[pagrunt]

I just swapped out back in May my 2.73 gear rear for a 3.73 and I’m already regretting it
Positives
Picked up half a second in the quarter
Negatives
Drinks fuel lost 2.5mpg
Sucks to drive on the turnpike and interstate
Constantly taxing the motor when driven
275/50/15 and 255/60/15 tried both the stock 205/70/14 are a joke
I have a set of 3.23 gears going to swap them out and do the apples to apples comparison
Engine is a stock 305 with summit camshaft edelbrock intake Holley 600 stock 1987 SS exhaust with cat Y pipe
15.3 @88mph

This is a reason why I'm running 2.73's with the Saginaw. No OD & having to do alot of highway driving to get somewhere I wouldn't go over 3.08's for a cruiser. I can make up some "performance" since I have a 3.11 first gear. If I was using the TH200 4R I have or a OD manual I'd be in the 3.42-3.73 range.

 

[Cuse99]

I'm going to be *that guy*....

But to buy a carrier, buy gears, pay labor to pull apart the existing rear and upgrade everything... assuming a shop will use your used rear parts from a liability standpoint, I just think you're money ahead thinking about the big picture. And best case scenario you just spent a pile of money .... on a 7.5

You be you 😊

I agree with your last statement, bu I might not buy a new carrier and all that stuff. Might just change the gears if I can. But I'll see if I an find a rear that is a direct swap but has disc brakes. Any idea or make/models I should look for?

 

[ck80]

You be you 😊

I agree with your last statement, bu I might not buy a new carrier and all that stuff. Might just change the gears if I can. But I'll see if I an find a rear that is a direct swap but has disc brakes. Any idea or make/models I should look for?

Do some reading about a rear gear change and setting the pattern, especially with used gears, first - then you know what you're getting yourself in for.

Lots of guys have documented and asked questions as they tried and did it here for the first time. I'll caution you, most of them tore it apart and together 3, 4 , 5 ,or 8 times together it right over a period of weeks. It's not something to be scared of, just something that takes time and has to be just right. And it's a very foul smelling job lol. So plenty to learn from.

I'd say worst case scenario is taking it apart and deciding its too hard, then having to transport the car sans complete differential to a shop to get done. Try not to wind up there.

You can modify a rear end, or buy a complete aftermarket setup with rear disc. In some cases you can buy custom control arms and suspension to switch something in. But if you're thinking "I'll use s10, or 8.8, or fbody, or ...." none of then can you drop the old one and just reattach shocks, brakelines, and the 4 control arms and be driving. you're buying some custom parts, etc etc.

You might find a gbody setup someone already modified to have rear disc that is used, but it wouldn't have been factory...

 

[pontiac guy]

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

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

 

[1320John$$$]

This is a reason why I'm running 2.73's with the Saginaw. No OD & having to do alot of highway driving to get somewhere I wouldn't go over 3.08's for a cruiser. I can make up some "performance" since I have a 3.11 first gear. If I was using the TH200 4R I have or a OD manual I'd be in the 3.42-3.73 range.

I have a 200R4 in my car

 

[64nailhead]

No one has asked, what rear tire is the OP running?

3 times I've put gears ins one of son's rides to have him change the rear tire height within 2 months. I'm done with them now - we are at pick a tire and wheel package and then I'll put gears in to match.

 

[pontiac guy]

No one has asked, what rear tire is the OP running?

3 times I've put gears ins one of son's rides to have him change the rear tire height within 2 months. I'm done with them now - we are at pick a tire and wheel package and then I'll put gears in to match.

He says he actually hasn't picked yet. That's in the future. Hopefully he decides on a tire size and then picks a gear rather than vice versa.

 

[Cuse99]

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.

 

[64nailhead]

Pick your tire size before you pay someone to swap gears.

A 3.73 is a great gear if you have 27-28” tires, but with a 25-26” tall tire it’s too much gear. If you ha e a stick motor that will cruise at 1500-1800, then you go front wheel. If you a giant cam in it that needs 2800 rpms to cruise, then obviously you will need a different gear.
These are things you have to decide before you pick a gear ratio.