Gearing Help

[Marcus]

What type of 350? A "run of the mill" stock 350? I'd recommend what HotRod was saying up above. Possibly 3.73s, but with your OD then I'd say 4.10s. This will give you all the power you'd need when towing in drive and still have a good lower RPM for highway cruising empty.

https://www.crawlpedia.com/rpm_gear_calculator.htm

There are also websites like this one if you want to be able to calculate RPM based on vehicle options/specs at different speeds.

A 31x10.50 tire will be around 30" tall.

So if you had 3.73 gears running empty at 70 mph then you'd be at 2,047 rpm (provided the TC is locked)
If you had 4.10 gears running empty at 70 mph then you'd be at 2,250 rpm.

I have no idea about the tc. All I know is the trans, tc, and transfer case were given to me and the trans and t case were supposed to have been rebuilt with tougher parts than factory. I like the idea of running between 2-2300 rpm so either of those would work. This is all in an 85 GMC High Sierra btw if that makes any difference.

 

[shiftpro]

If you're going with an OD transmission, especially a 700r4 and you're gearing, I'd go with 4.10's being a 4x4. Lower the better, meaning higher numerically. You'll have to OD to compensate and if you're gears are to tall, it's a bigger strain on the transmisison. You don't need OD with 3.08's. 3.73's would be the tallest I'd even consider with an OD or you won't get to take advantage of the OD so figure 3.73 or better like 4.10's maybe even 4.56 but if you go 4.56 I think you need to swap carriers too so to keep it cheaper I'd consider 4.10's.

I look at OD transmissions as a 4 speed... meaning, I'm not thinking about OD dropping a bunch of revs on the highway and basically or nearly lugging the engine. I want 3rd gear to be on the revy side at highway speeds, not in the comfort zone you would normally aim to gear for with for example, a TH400.

Again, just FYI, 35" tires and 4l80 = 4.88s... or LOWER if it's a heavy truck.... meaning one tons.

 

[Frankenchevy]

Just to add more to the mix, I have 3.73s, a Th400 and 33s on a one ton truck and I’m happy.

 

[77 K20]

I have no idea about the tc. All I know is the trans, tc, and transfer case were given to me and the trans and t case were supposed to have been rebuilt with tougher parts than factory. I like the idea of running between 2-2300 rpm so either of those would work. This is all in an 85 GMC High Sierra btw if that makes any difference.

Someone else can chime in here- but with a stock 700R4 torque converter they can lock up when cruising. This prevents a few hundred RPM of torque converter slip. Makes it more like a direct drive manual transmission. I had an 86 blazer many years ago and if I remember right it would only lock up when in OD. Under some light load it would feel like a shift, but it was only unlocking the torque converter.

 

[HotRodPC]

Someone else can chime in here- but with a stock 700R4 torque converter they can lock up when cruising. This prevents a few hundred RPM of torque converter slip. Makes it more like a direct drive manual transmission. I had an 86 blazer many years ago and if I remember right it would only lock up when in OD. Under some light load it would feel like a shift, but it was only unlocking the torque converter.

That is correct. It all depended on the pressure switches in the transmission. Some only locked up in OD but some applications did lock in 3rd of D too. Like if you're cruising in the city doing say 40mph, it would lock up in 3rd too. Personally I like lock up in OD only and it does save 200-300rpm.

 

[Marcus]

This is the truck I’m working on. Have done a few things since this picture but haven’t gotten any new pictures. Work in progress for sure.

You must be registered for see images attach

 

[HotRodPC]

Certainly worthy of working on. That thing looks straight.

 

[Marcus]

She’s fairly solid. Has a few rust spots that need attention but nothing major. Just needs someone to love on her is all. Used to be my grandpas truck then my uncles high school truck. So she’s special.

 

[HotRodPC]

She’s fairly solid. Has a few rust spots that need attention but nothing major. Just needs someone to love on her is all. Used to be my grandpas truck then my uncles high school truck. So she’s special.

Oh yeah. That makes it very worthy of spending some cash and time on it and make it right. It don't look all that bad now for it's age.

 

[Marcus]

No just needs a good bath and some parts replaced and or fixed. Don’t know about the mileage on her. Gauge shows 55k but I’m gonna go ahead and guess that’s probably 155k lol

 

[4WDKC]

I've seen some 2.73 front axle gears in 10 bolts. In fact I have one sitting in my garage right now that will be getting converted to 4.10 when all said and done. I think they were typically found under diesel rigs but I am no eggspert on what came in what (gas or diesel). Imagine how much better the diesel rigs would have been if they didn't have **** gear ratio's.

That being said with a 35 inch tire you are going to be wanting to be in the range of a 4.56 gear with your LS motor and NV4500, otherwise I'd be surprised you can find a use for anything over 3rd gear. Probably burning a **** ton of fuel just because of not being in a decent gear and at peak efficiency...

35s and 410s will cruise down the interstate around 2000 rpm depending on you OD ratio, my truck with a 700r4 and .70 did this. It wasnt a bad combination but wasnt a rocketship either. I think you would be happy with this.

Okay so I’m putting in a 350 in my GMC. For some reason it has a .308 rear end gear in it. The front is factory .342. I’m going to putting 31x10.5 tires on it and also it’s getting a 700r4 trans. So what gear would be best? Cause that chart and all I’ve read I’m confused because people are saying way too many things.

Loose calculations this will put you at 1800 rpm, its may have to come out of lock or down shift to climb hills or pass someone on the interstate. a 3.55-3.73 would be better with your tire size.

the down side to both of these is you have to swap the front axle also or you cant use the 4x4 without breaking your transfer case. and that can be expensive. What I did was find axles that already have the gears you want and swap in. This was cheaper for me as it cost me about $500 for the swap instead of $1000, if you know someone that can do the install the price will be alot closer.

 

[Marcus]

35s and 410s will cruise down the interstate around 2000 rpm depending on you OD ratio, my truck with a 700r4 and .70 did this. It wasnt a bad combination but wasnt a rocketship either. I think you would be happy with this.



Loose calculations this will put you at 1800 rpm, its may have to come out of lock or down shift to climb hills or pass someone on the interstate. a 3.55-3.73 would be better with your tire size.

the down side to both of these is you have to swap the front axle also or you cant use the 4x4 without breaking your transfer case. and that can be expensive. What I did was find axles that already have the gears you want and swap in. This was cheaper for me as it cost me about $500 for the swap instead of $1000, if you know someone that can do the install the price will be alot closer.

So why would I have to switch out the front axle just cause I switch out the gears? I’ll be switching both front and rear to match.

 

[77 K20]

You don't have to switch out axles, he was just saying that can be a less expensive option- find used axles with the ratio you want. It is often less expensive than a shop installing new gears into your existing axles.

 

[Marcus]

Okay. I can do the work myself. Just was curious on what everyone thought about on the gearing. Everyone that gave me supportive help, I appreciate it. I think I’m going to go with the 3.73’s.

 

[rpcraft]

Pictures or it didn't happen!!!