Will 26 inch tires grip with a 4.10 if I have a posi? I took a posi out of a trans am. Trying to choose 3.73, 3.90, or 4.10. Having a hard time deciding. Don't really care about the mpg. I do a fair bit of highway driving where i live but nothing real long distance just 10-20 miles each direction a few time a week. I don't mind buying the gas but I don't want to burn up my motor premature. So I guess there is more than one question in my post but the main one is whether or not 4.10 should even be in the running. I just bought a set of tires so I am going to have them for a couple more years.

 

I dont see a reason to go more than 3.73 in a 700r4 with most motors.

Tire type and suspension will determine if they will hook. Mickey thompson drag radial will hook. Your cheapo all season types with 400+ tread wear rating may not. All depends on the type

 

Okay, Most of my suspension is still stock. I have replaced bushings with after market but springs, struts, steels are all stock still. My tires are not el-cheapos they are directional sport tires but I'm sure they are radials. I think they are Kuhmo's. Engine is moded, but nothing amazing. I think I am somewhere between 350-400hp. No dyno or track numbers b/c I am still not satisfied with it. People ask me to race and I just tell them it's not ready yet. I feel like getting rid of the 2.73 gears will make me much happier. Does any of this info help? I will eventually upgrade every piece of the car, suspension, springs, struts, but right now I'm still trying to get it running the way I want it. I have read a lot of places that 4.10's have a much harder time sticking than 3.90's but I am wondering if those guys are still running the one wheel wonder peel or if putting this posi in my rear will give me enough hook up. Also I plan to upgrade to a locker in the future but right now I have this trans am posi sitting here waiting to be installed.

 

I was thinking about 3.73 b/c they are so popular but, you know, I just wonder how much quicker a 3.90 or 4.10 would make it and just don't want to be disappointed if I go with the 3.73's.

 

4.10 is going to make 1st gear useless with the 3.06 ratio in the 700R4. 3.73 would be better. Engine specs also have a lot to do with what gear is best.

What differential did you get out of the TA? Is it a used Auburn? If it is then it might not be worth the time to put it in. They are not great when they are new and usually junk when used. You also need to know what series it is so you can get the correct thickness of gear for it.

 

Problem is with 4.10's you may run out of rpm and be forced into overdrive at wot when at the track or when street racing. Best gearing usually puts the motor at its peak hp rpm, or slightly higher when crossing the 1/4 mile marker. Depending on where your peak rpm is, and vehicle weight, 3.42-3.73's seem to work well. Higher rpm motors use the 3.73-4.10 stuff unless they also have real high hp and high mph speeds. Then they back down in gear.

You are right 4.10's could be harder to hook due to torque multiplication. More gear is more force at the tire.

No need for a locker a good posi is all you need for traction. Traction will be based on tire and suspension. Most street tires however will spin in lower gears.

 

I got the posi out of a third gen Trans am. It's either a 7.5 or 7.625, I'm sure it's stock. I'm just building a street strip car. I know the small gm 10 bolt, that is in these third gens stock are not very strong but it's what I have and I'm not trying to wait til I have the money to upgrade my whole rear end before I upgrade my gears. Sounds like I need to go with the 3.73's tho'. Basically all of you guys said 3.73's over the 4.10's, same opinion over the 3.90's? I'm thinking you're gonna say yes. So it looks like I will more than likely be going with the 3.73's.

 

That's going to be the Auburn. You still must know what series it is before you can order any gears.

 

The 7.5/7.625" ring gear isn't really big enough to handle all the teeth in the 4.10 ratio. The higher the ratio (numerically) the more teeth on the ring gear. The more teeth, the weaker the gear. 3.73 is considered about as high as you want to go to maintain reliability.

 

Thanks guys. I really appreciate it. I'll be going with the 3.73 after hearing from all of you. I am just going tot ry and pull one out of a blazer or something at the junk yard and save myself some money since 3.73 were pretty common in those. I'll just wait till I upgrade my rear end before I buy some quality gears. My car shoves me and my sons back in our seat on launch even with these 2.73's in it so I am sure 3.73's will feel great. Thanks gain for all of your input.

 

Again, what series is the Auburn that you pulled out of the TA? If it's a 2 series then the Blazer gears will not work.

 

If you can tell us what gear ratio came with the posi unit out of the Trans Am then we can tell you what series the carrier is. What big gear head is trying to tell you is that you need to know what series carrier you have because that will determine/influence the entire install.
I believe the year of the Trans Am is also important to determine whether or not it was 26 or 28 spline axles.I think if you count the splines on the side of the carrier/posi it may tell us what spline axles it needs.

 

Okay, sorry I have taken so long to get back to you guys. I have been extremely busy recently. So how do I determine which series this diff. is? I did not take the pinion gear but I still have the ring on the diff. It is stamped with the year 1987 on the carrier, don't know if that helps. Let me know how to determine what series it is and I will let you know asap. Thanks again.

 

Post the numbers that are stamped into the side of the ring gear. That will tell you what the gear ratio was and that will tell you if it's a 2 or 3 series.

 

Going from2.73 to 3.42 ratios with Eaton posi.in my little 5.0, auto. made a huge difference.It feels like I added 50hp.A night and day difference. Other than replacing my engine with a stronger 350 it is the best and most productive modification you can do. Just through a Corvette servo in your trans. for firmer shifts and you're good to go with a car that feels totally different and a major improvement.

I believe that the 700r4 trans. works best with the 3.42 in my car because I like having a longer first gear than the 3.73 offers.

You’re going to love the 3.73s
Side note = If you referring to a 275/60r15 tire in the title of this thread than it is a whopping 28in.tall.

 

Heck, if it takes off that good with just 2.73's in there, I'd say a set of 3.42's would be more than enough to make you happy. But if you're set on 3.73's, those'd be ok too I guess. You just won't have a very long 1st gear. But yeah, ditching the 2.73's is really a must-do mod. They suck!