Differential gear ratio questions

[Thundergun]

i have 3.73's with a limited slip out of a suburban in the rear. I have 33" tires and a 6" lift. It's great. Great response off the line and feels like good top end.

I don't have a tach, but the motor def is not screaming. it feels good at 65 mph. Then again i don't drive it at 70 or 75 so i can't say how that would be, especially daily if you had to. This is just a weekend cruiser truck. I could easily stand 4.10 or 4.56 ... i think ...

But that's my story and im stickin to it. 

[Magic1]

Lol. Think I'm gonna try the 3.73s. New truetrac rear diff with fresh bearings and call it good. Appreciate all the input. Be nice if it didn't cost so much and ya could experiment a little but I guess you gotta pay to play right?

[fitz]

I vote for 4.10's.
My 81 stepide has 4.10's with 35" tires. With a turbo 350 trans this seems like a great set up. I haven't drove the truck in years, so I can't remember the rpm's at highway speed. It just felt right.
My M1028 has 4.56 gears with 285/75R16 tires (32" +/-).  With a turbo 400 trans this truck is good for about 60 mph on the highway.  Any faster, and the 6.2 diesel sounds angry. 
Is your truck running 10:bolt axles?

[Magic1]

10 bolt up front and 12 in the rear. Not the strongest setup I know, but will work fine for what I need.

[roundhouse]

3.73 is very common
Should be able to find that in a used axle if you feel
Comfortable swapping the whole axle

I think 3.73 would be a good choice
We run that with 35 inch tires


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[Magic1]

Man...just when I think I may have things figured out my wife throws something else in the mix. She brought up pulling a camper to the lake. That's got me wondering if the 3.73s would be good enough or if I'd be better with the 4.10s or would there even be any real advantage one over the other. I'll definitely add a auxiliary transmission cooler.

[Irish_Alley]

typical lol just when you thought you had it. but how big of a camper are we talking? how far of a travel? hills?

[Magic1]

typical lol just when you thought you had it. but how big of a camper are we talking? how far of a travel? hills?

Like a 30ft travel trailer that would be left at the lake. Not pulled back and forth but only occasionally. I live in Kentucky so it's fairly hilly. I'd say probably around 50-60 miles one way.

[pholliday1]

You'll most likely end up disappointed with anything less than the 4.10. Unless your motor been massaged our small blocks are usually just decent on power throw in a taller tire and it's like losing 50hp.

[Magic1]

You'll most likely end up disappointed with anything less than the 4.10. Unless your motor been massaged our small blocks are usually just decent on power throw in a taller tire and it's like losing 50hp.

Are you talking about while towing or just in general? See that's what has me concerned about going to such a lower gear like a 4.10 or higher. It is slow on take off but really not bad going down the road with the 3.08s. I've got a crate goodwrench 350 with an edelbrock performer manifold and headers. Supposedly it's making around 260 hp but who knows. I want better towing ability and basically just want the truck more usable and fun to drive but not be stuck in permanent granny gear. Looking at gear calculator charts online say with a 35" tire to get it back to stock I should go with a 4.10-4.88 but it doesn't specify whether or not that's with an overdrive transmission. I've got the th350 and don't plan on going to the 700r4. If I'd planned a little better or knew more about what I would be doing when I bought the truck I would have got one and had it beefed up but I already had the th350 rebuilt with a mild shift kit and converted the old np203 over to a 205. I could sell the combo and get the overdrive but I don't want to start all over again and I'm satisfied with the way it works now.

[timthescarrd]

All the charts I've seen online are based on a 1:1 transmission ratio, which is what you're TH350 has in 3rd gear with the np205 in high. Considering a 35" tire is really around 34.5" at 65MPH you're 3.08 should be around 2000RPM switching to 4.10 would put you around 2650RPM, 4.56 would put you at almost 3000RPM.  3.73 at 65MPH is around 2400RPM.

I would say, go with the 4.10s as it's going to get better into the torque curve and still stay reasonable on highway

[pholliday1]

4.10 )

[Don5]

I also vote to go with the 4:10. It will help you when you are towing. The truck won't be a slug with a camper behind it. I would also suggest a transmission cooler if you are going to tow with your truck. That is just cheap insurance.

[Irish_Alley]

yeah with the trailer and not having a manual trans i have to change my vote to 4.10

[roundhouse]

If your towing is just a one time deal
Install a good Trans cooler and haul your camper
Just don't get in a hurry and remember it's better on the engine and the Trans to wind it up fast than to lug it so you may need to drive some or most of the way in second gear depending on the hills and how bad your trailer sways


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