3.42 gears vs 3.73 gears

[Marcus]

Okay so I have an 85 GMC K1500 High Sierra that originally had a 305 and 3.42 gears in it. Someone blew the rear and swapped in a rear end that has 3.08 gears in it and took the front drive shaft out. Was curious on what gears would be better with the 350 I’m putting in it, the 3.42 or the 3.73? Stock suspension, stock rims, 31x10.5r15 tires, and 700r4 trans. Wanting power without lag throughout all speeds and also keep a decent mpg as possible. Obviously it’s not gonna get 20mpg but would like to get around 15mpg if possible. Any input would be helpful.

 

[dvdswan]

It depends on what you will be using the truck for. Highway driving I go with the 3.08, city 3.42, and towing 3.73. just my $.02.

 

[4WDKC]

342 and a 700r4 will give you 1900 ish rpm at 70mph, 3:08 is 300rpm less, 3:73 is 300 more... if you have any plans for bigger tires 373 is a min. 410 + are better for big tires

 

[yevgenievich]

At least 3.42, and 3.73 works well for a stockish k10

 

[WFO]

If you're stickin with 31" tires, and your front end is already 3.42, I'd go with 3.42 in the rear too.

 

[Marcus]

That’s my thought is to stay with the 3.42 to save a little money right now. I’ll never put a lift or big tires on it. It’s kinda a tribute truck. It used to be my grandpas truck and then it was my uncles hs truck and then my cousins hs truck so I want to keep the build simple and stock mostly. Just want decent power and mpg all through. The 3.08s suck until you get to high speed so those won’t stay either way. Now looking through the old trucks and jimmys out in the field for 3.42 gears. Thanks guys.

 

[Marcus]

It’ll be a mix of city and highway with hardly any trailering/hauling. May pull a small trailer with a mower on it once in a while but that’s probably about it.

 

[Camar068]

Now that I went from 3.08 to 3.73, I'm happy. Since driving the 73's I've wondered about what if I'd done 4.11 or 3.42. I almost want 3.42, but with the hill/mountains around here I think I'd regret it. The 4.11's, the rpm's would be too high for me driving to work everyday on the interstate.

I'm really thinking about putting 33" tires on it and see how that works. I'm guessing that would be the equivalent of 3.56/3.42 gears. [edit] disagree's with stated above, but tires can be changed lol.

At 80 MPH on the interstate, I don't have an issue with passing traffic....all light throttle and no downshifting going up hill. Honestly, the engine wants to run at ~2500rpms which is about 85 mph. I'm worried about getting a ticket lol....and I've never had one in my life.

 

[Camar068]

couple good calculators I've found. i just now found the second, which makes me rethink my gear selection (no I don't wanna hear your **** @HotRodPC lol). Yeh you told me.....if I go up to 33" in tire it's not gonna be in the sweet spot. Run 3.08, 3.73, and 4.11 with either 28.75 (31") or 33 (31.5") tires in the calcs below.

You'll see the rpm comparison I'm talking about. My engine wants to take off at about 2,400 rpms with current setup.


https://www.intercotire.com/tire_size_change_gear_ratio_calculator

https://www.randysworldwide.com/calculators/rpm/

 

[HotRodPC]

couple good calculators I've found. i just now found the second, which makes me rethink my gear selection (no I don't wanna hear your **** @HotRodPC lol). Yeh you told me.....if I go up to 33" in tire it's not gonna be in the sweet spot. Run 3.08, 3.73, and 4.11 with either 28.75 (31") or 33 (31.5") tires in the calcs below.

You'll see the rpm comparison I'm talking about. My engine wants to take off at about 2,400 rpms with current setup.


https://www.intercotire.com/tire_size_change_gear_ratio_calculator

https://www.randysworldwide.com/calculators/rpm/

I know I didn't say anything about running 3.08s. IMO for 31-33in tires, 3.42 or 3.73's are good if you're running a non OD transmission. If running a 700'r4 then my preference would 4.10's or 4.56's if found a set of axles but not worth a gear change in 10bolts since you'd have replace the carriers too. I don't tell anyone what to do, I just suggest what I'd do. I think my K10/20 is going to end up with 4.10's 32's or 33's and 4.10's. I'll just have to keep it around 60 on the hwy. It's more of a fun truck so mpg and getting somewhere in a hurry doesn't really matter. All vehicles have a purpose. Gear it and Tires accordingly.

 

[4WDKC]

couple good calculators I've found. i just now found the second, which makes me rethink my gear selection (no I don't wanna hear your **** @HotRodPC lol). Yeh you told me.....if I go up to 33" in tire it's not gonna be in the sweet spot. Run 3.08, 3.73, and 4.11 with either 28.75 (31") or 33 (31.5") tires in the calcs below.

You'll see the rpm comparison I'm talking about. My engine wants to take off at about 2,400 rpms with current setup.


https://www.intercotire.com/tire_size_change_gear_ratio_calculator

https://www.randysworldwide.com/calculators/rpm/

410s and 35s is similar rpm and speed on the interstate as stock tire size and 342 gears.

 

[Camar068]

I know I didn't say anything about running 3.08s. IMO for 31-33in tires, 3.42 or 3.73's are good if you're running a non OD transmission. If running a 700'r4 then my preference would 4.10's or 4.56's if found a set of axles but not worth a gear change in 10bolts since you'd have replace the carriers too. I don't tell anyone what to do, I just suggest what I'd do. I think my K10/20 is going to end up with 4.10's 32's or 33's and 4.10's. I'll just have to keep it around 60 on the hwy. It's more of a fun truck so mpg and getting somewhere in a hurry doesn't really matter. All vehicles have a purpose. Gear it and Tires accordingly.

hehe, just razzin ya man. Tryin' to say you were right lol....and was good advice.

 

[Marcus]

Okay so I’m also curious on why I have a non Posi Rear end? Only one tire spins when both are off the ground and I turn a tire. Why is it that way when it’s supposed to be a 4wd truck? I’m not the most educated person but that just don’t make any sense to me.

 

[HotRodPC]

Okay so I’m also curious on why I have a non Posi Rear end? Only one tire spins when both are off the ground and I turn a tire. Why is it that way when it’s supposed to be a 4wd truck? I’m not the most educated person but that just don’t make any sense to me.

It's all 4 wheels are cabable of being powered. Doesn't mean they are all going to work at the same time. To do that you'd need lockers in the front and rear axles. Most don't have posi or lockers.

 

[yevgenievich]

Okay so I’m also curious on why I have a non Posi Rear end? Only one tire spins when both are off the ground and I turn a tire. Why is it that way when it’s supposed to be a 4wd truck? I’m not the most educated person but that just don’t make any sense to me.

Rear locker was optional, and in a 10 bolt stock locker was not very strong and most prefer open diff to install aftermarket locker. Front was left open regardless unless maybe some military offerings. But selectable lockers or posi can be installed in the front as well.