what gears should I run in my 3/4 ton lifted gmc?

[marks86]

Lol how do I determine that I have 3.73 gears without opening everything up? Im pretty sure that the year and axles I have came with 3.73s.

do you have a tag in your glovebox with all kinds of useful info

 

[ruedger455]

Im not sure I havent checked. Would it be on there?

 

[Christian Nelson]

I would avoid 4.56... even 4.10 for the cost on a DD.. The reason you aren't able to spin the tires is you are driving a full time 4x4 truck! You put some locking hubs on the front, and stick the transfer case in hi-loc, I think you could spin the 35's with your current ratio, maybe not if you have 3.55, or 3.05.. But, if you have 3.73 you should be able to..

I was able to with a TIRED old 350 in my old mud beater, same setup, with 36" Buckshot mudders. TH350 tranny, and 203 transfer case. I just put locking hubs on it, and drove it in hi-loc when the hubs were in "free" position. It got better gasmileage, handled more agile-ly, and I could spin the back tires on it on dry pavement..

If that's all you wanna be able to do, just buy a set of WARN locking hubs, a whole lot cheaper, and you still won't burn rubber with all 4 wheels on dry pavement, even with 4.56, unless you put it in low..

 

[ruedger455]

I have locking hubs on it. Maybe it doesnt have the 3.73s like I was told.

 

[350runner]

What are your. Highway rpms? Might help us guess the gear ratio

 

[MadOgre]

I was talking about spinning them off road.

 

[ruedger455]

I coukdnt tell you what my highway rpms are. I dont have a tac. But id say they are around 2500 at 70 if I had to guess.

 

[ruedger455]

This number is stamped on my 14 bolt ff 3877323

 

[Cowboy Cadillac]

I run 4.10s and 35s with a T350 & NP205....screams at highway speed, around 2600-2700 rpm. I can spin the tires easily on gravel, even in 4wd, with a basic unadulterated 350 4bbl.

 

[Chaz]

I think that rpm chart is off 4lo.com,,,,its just a math chat based on gear ratio/tire size/and 1 to 1 top gear. It's about 200-250 rpm between 3.73 to 4.10 and 4.10 to 4.56 ratios,,,

Was runnin 4.56's for about a year after I got my burban with th400 trans,,,I loved those gears,,it felt like there was never any real drag on the motor,,,it would accelerate in traffic/off road/mud/up hills almost effortless all the time,,it just ran out of gear real quick, I never tried to spin the tires on the street though lol,,,I kept it as my daily driver and drove from la to san diego a few times,,,I could barely do 65 at about 3000-3100 rpms so it was screamin but still had no real problems except it burnt up a hell of a lot of fuel lol But what I really didn't like was I felt like the excessive rpm's was gonna wear out the engine really quick.

The rear end finally went bad so I swapped in a 14bolt ff with 3.73,s. Its actually not too bad on the highway now. I can do 75 mph if I have to and still cruise at 65. In traffic it still accelerates good. The motor feels really strong. It still pulls the big burban around fine. I think the motor likes the 3.73,s better because it feels like the motor was made for low rpm torque and now its being used the way it was designed. Gas mileage is also way better. Based on my gps if im doing 65 mph my speedometer is showing 60. And it is consistently off by 5 mph from 25 up to 65mph. So I believe im still 5 mph undergeared from whatever stock gearing was. Prob had 3.73,s org with stock tire size. Now I want to go to 4.10's even though I know its gonna hurt mpg and prob overgear it a little from what stock gearing was.

 

[MadOgre]

I think that rpm chart is off 4lo.com,,,,its just a math chat based on gear ratio/tire size/and 1 to 1 top gear. It's about 200-250 rpm between 3.73 to 4.10 and 4.10 to 4.56 ratios,,,

Was runnin 4.56's for about a year after I got my burban with th400 trans,,,I loved those gears,,it felt like there was never any real drag on the motor,,,it would accelerate in traffic/off road/mud/up hills almost effortless all the time,,it just ran out of gear real quick, I never tried to spin the tires on the street though lol,,,I kept it as my daily driver and drove from la to san diego a few times,,,I could barely do 65 at about 3000-3100 rpms so it was screamin but still had no real problems except it burnt up a hell of a lot of fuel lol But what I really didn't like was I felt like the excessive rpm's was gonna wear out the engine really quick.

The rear end finally went bad so I swapped in a 14bolt ff with 3.73,s. Its actually not too bad on the highway now. I can do 75 mph if I have to and still cruise at 65. In traffic it still accelerates good. The motor feels really strong. It still pulls the big burban around fine. I think the motor likes the 3.73,s better because it feels like the motor was made for low rpm torque and now its being used the way it was designed. Gas mileage is also way better. Based on my gps if im doing 65 mph my speedometer is showing 60. And it is consistently off by 5 mph from 25 up to 65mph. So I believe im still 5 mph undergeared from whatever stock gearing was. Prob had 3.73,s org with stock tire size. Now I want to go to 4.10's even though I know its gonna hurt mpg and prob overgear it a little from what stock gearing was.

Great info! What size tires are you running ?

 

[ruedger455]

Well I wanna thank everyone that took time out of their lives to help me out with vital information. Ive decided that, im going to go with 4.10s. Depending on what my gear ratio is right now. If its 3.73 or higher ill go with 4.10s. Thanks guys I appreciate all you do.

 

[Chaz]

Im runnin 35's BFG AT KO, they actually measure at 34 inch diameter mounted. I heard someone else on here said there runnin same setup as me with th400 and they like the 4.10,s for DD.

 

[MonsterZ]

Maybe it's just me, but I really hate band aid cures. I'd do a little homework first, like pop off your diff plates and find out for sure exactly what you have first. I'd then also get ahold of that dyno sheet and find out for sure what you are making for power and where in your power band you are making it. If you're not making serious ponies until you are over 3,000 rpm I'd likely as not consider a cam swap and put your torque band back down where you use it. After that I would look at exactly what you decide to accomplish with your truck and build for that. If you are making decent torque and horse power between say 2,000-4,500 rpm, and you have legitimate 3.73 gears, and you are still seriously lagging with 35's, go with the 4.56's, especially if you plan on going up to 37's. Understand that flywheel power and at ground power are entirely different and that using a rough 20% reduction is helpful, but not precise. The more you add in drivetrain, the worse that ratio gets, and exponentially so. Re-gearing is simply an effort to put you back in your engines power band. If that's not enough, consider building your power band. Consider the fact that there are plenty of 400+ hp motors that don't make even 80 hp under 1,200 rpm, because that hp peak isn't until 4,850 rpm.

 

[HotRodPC]

It's sure interesting to see all these different opinions. All seem valid and bring something positive to light even though they're so different.

I do agree to find out EXACTLY what you have FIRST. This way you can compare apples to apples. In 77, 4.10's were pretty common in the K20's, and 3.73's were next as far as common. It would be a shame to regear to 4.10's to find out that's what you have in it already.

If you do have 3.73's already, I don't see 4.10's giving you the performance you're going to be expecting. Basically, all you're doing is adding a little over 1/4 turn, increasing driveshaft turns to 4 full turns from 3.75 turns. See what I mean? Very little difference and not worth the expense IMO. So if you're looking for a performance gain based on gear ratio and paying to have a gear swap, then I'd at least go to 4.56's and I'd be OK with since you mention you don't or very rarely drive it on the freeway. For the occasion that you do, so what the motor may be screaming at 3,000 cruising rpm. But then you can get into more expense to go 4.56's too. Cut off on the carrier for a 14b FF is 4.56 on down, and 4.10 on up, so to go 4.56's you'll need a different carrier and more expense. Dana 44, I think you're still OK on the carrier split IIRC.

Regearing isn't cheap, especially considering you have to do 2 axles. What you might want to do if you do indeed find out that you have 3.73's, is just keep your eyes open for a set of used 4.10 axles. If and when found, you can usually pick up the complete sets ready to bolt in for $300. Sure they're used, but 14b FF rarely go bad, and if they say the Dana 44 or 10 bolt front is in working order, good chance that it really is. You'd want to pull the covers anyway and spin the gears around and look for shipped or broken teeth. So that's my big suggestion. #1 find out what ratio you have now. If it's 3.73's like you think, then just keep an eye out for some working condition used 4.10 axles. Then you also have a spare set of axles that if you choose, you can probably sell the for what you paid for your 4.10 axles, or keep them for spares. They don't take up that much room, out behind the shed, and lean half sheet of plywood or tin siding up against them to keep the majority of the weather off of them.

Someone brought up 2 points. That 35in tires aren't actually 35, they are 34 when mounted and on the truck. This is true, but with radial tires, they do grow with centrifical force and they will grow to at least 35in, maybe even 35 1/8 inch at freeway speeds. Also brought up the chart is lying and not taking into consideration torque converter loss. And that is true. Unless you've got a lock up converter, that chart would be 200-300 rpm off in top gear of any automatic transmission.


Someone else brought up not being able to do a burnout due to being Full Time 4x4. That is probably true too. And in all honesty, wanting lower gears just to be able to do a burnout isn't really practical. Especially without having and OD, you should be wanting running the tallest gear possible based on what and how often you'll be towing something unless its a competition vehicle. You've got a fairly strong transmission so not much need in trying to take the strain off it like we need to do for a 700r4. So even the 3.73's are practical for you. Keeping in mind, an NP205 is almost a 2:1 crawl ration is 4 LO LOC, that now makes your final ratio real close to 7.46:1 to the rear wheels. So if you're climbing a steep hill or pulling a tree stump, that 7.46 is going to be to far off of just over 8.20 if you had 4.10's. All you're looking for is enough gear to get the stump out or up the hill, no reason for overkill. Even guys running 3.08's in K10's can get by because when they get off-road they can multiply the ratio by 2 with a NP203 or NP205 to a right at 6.00 crawl ratio and the later trucks running NP208's are getting to multiply that 3.08 by 2.6 IIRC.

Just putting that all out there for consideration. I'd hate to see you spend bookoo bucks and not get the satisfaction you were expecting.