ReGearing Questions - Noobie
 

Hey I have a 1985 GMC 305/700r/208np and currently is geared to 3.08 , I have GM 10 bolts, but ive been told thats pretty much the same axle as the Danna 44? idk. Anyways I have a 4 inch lift and 33s right now, and the power is pretty good, tops out at 90 or so but this truck is my daily driver so keep that in mind. I'd like to put 35s on it and was curious on what i should regear it to, 1. for power spinning those tires, 2. MPG if any of yall could point me in the right direction... Thank you!

Re: ReGearing Questions - Noobie
 

if you go with 35's, i would put some 3:73's in there. your 305 will thank you, & your 700 should keep hiway rpm's decent.
i had 3:73's in my k5 with 35's, could chirp the tires, 400 sb though. with a t350, revs weren't the best
my stepper also has 3:73's with 33's, 350/5-speed, easily spin the tires, gets about 15 mpg with the overdrive

Re: ReGearing Questions - Noobie
 

Awesome! Thank you for the response, I'm just trying to gather some information on all this, but after reading that ill mostly go with 3.73s

Re: ReGearing Questions - Noobie
 

I'm running 35's on the '63 with an overdrive manual (.73 Od) , 396 and 4.10 gears. Freeway speeds ( no speedometer yet but around 75-78) is around 2100-2200. Still barks 3 real hard and gets 4th easily. Tows real well and gets decent ( for a big block) mileage. 3.73's wouldn't be bad either. I think the 700r4 has a pretty deep 1st gear IIRC.

Re: ReGearing Questions - Noobie
 

I have 3:73's and 35's behind my th400 (no OD) and at 3 grand I'm doing about 60mph

Re: ReGearing Questions - Noobie
 

That sounds pretty good, Right now it cruises at 55-60 at 2000-2100 rpms in overdrive. all the highways round here are 55mph (sucks) but i honestly feel like the 3.73s would be pretty nice match with the 700r and 33s/35s. Where did you get your gears? where did any yall get yours?

Re: ReGearing Questions - Noobie
 

Forgot to ask, Where did you get your 3.73s??

Re: ReGearing Questions - Noobie
 

well, i live north of the border, so that's probably not much help.
my stepper came with factory 3:73's, my k5 i bought Yukon ring & pinions from Lordco (our version of autozone, o'reilly's, whatever)
http://www.yukongear.com/ProductDeta...px?ProdID=5309
http://www.yukongear.com/ProductDetails.aspx?ProdID=541
there one of the pricier ones out there, but NEVER had a fail. Motive gear, & Richmond have better prices on theirs.
could prob. pick up a set off ebay or something for $2-$300

Re: ReGearing Questions - Noobie
 

With a 305, I'd run 4.11s for sure. You'll probably get better gas mileage because you'll be running in a better part of the powerband.
'63GENIII and I both have 100 cubes on you so that's probably why 3.73s sound feasible to us.
In your case the auto trans and torque converter end up picking up a lot of the slack for bad gearing. I stand by my recommendation for 4.11s

Re: ReGearing Questions - Noobie
 

That makes alot of sence. Than you, where do you suggest to look for 4.11s for a GM 10 bolt?

Re: ReGearing Questions - Noobie
 

I agree with Jason at 4.11 minimum. I'd even go 4.56 if you are never going much over 55. My 91 has 4.56's, 33" tall tires and an overdrive. It feels great with the lower gears and isn't too wound out at 55 with the OD. Again, your 305 will thank you.

Re: ReGearing Questions - Noobie
 

I drive it daily and drive it at 50-65mph, seems like what your saying 4.56s might be my best bet, I'm just now understanding the purpose of regearing, but would 4.56s get better mpg than the 4.11s?

Re: ReGearing Questions - Noobie
 

Snow,
Logic would tell you that a numerically lower gear, and larger diameter tires would result in lower engine rpms at a given vehicle speed, and thus better fuel mileage. However if you gear it so that the engine is spinning an an rpm that is too low on the horsepower curve it will actually consume more fuel, because it is making power far less efficiently than at a higher rpm. Each motor has a sweet spot that is optimum for power and economy at sustained operation. This is what the engineers determined when they designed out trucks.
In general, smaller displacement motors make power at higher rpms, than large displacement motors. so the gearing is usually numerically higher to put the motor in the sweet spot. So sometimes, when you've mixed and matched engines, transmissions, rear axles, and have a big heavy vehicle, it can be beneficial to numerically increase the gear ratio to get better power and better fuel efficiency.

An example would be my F0rd Bronco. I had 33" tires on it with 3.55:1 diff gears and overdrive with a small block. I recent put 31" tires (close to stock size) back on it, and my fuel mileage has gone up by at over 1mpg. When you're getting 13mpg, that translates into an 8% increase. If I want to run 33" tires again, I really should install 3.73:1 gears in the diffs. Make sense?

Re: ReGearing Questions - Noobie
 

Did your pickup come stock with 305/700r4/np208 and the 3.08:1 gears?

Re: ReGearing Questions - Noobie
 

Ahhh Okay, I believe I got it. So basically what you're saying is by increasing the gear ratio makes up for the larger tire size? Bringing the vehicle back to a "Factory" set up. I imagine when the 35s on on the truck and the correct gear set up my mpg may actually increase? well Thank you for the detailed response. Like I said before I'm still quite new to all this. The 4.56s are my best bet it seems. But yes it did come stock with 305/700r4/np208 and the 3.08:1 gears.

Re: ReGearing Questions - Noobie
 

I believe stock tire size that would have come on the pickup is a 235/75R15, which is about 29" in diameter. Now you want to step up to a 35" tire

So 3.08 original gear size / 29" original tire diam * 35" new tire diam = 3.72 theoretical new gear size

Then take into account those 35" tires are much wider, and heavier, and are probably a less efficient tread pattern. This takes more power to spin and overcome friction. So we'd bump up from the 3.73 we calculated, to a 4.11.

This will probably get you back to similar performance as what you originally had.

So now consider how you will use your pickup. Will you be hauling much payload, or towing much? If so you might consider MTCK's recommendation and go up more to a 4.56 gear ratio to put the motor into a stronger part of the powerband at cruising speeds.

Re: ReGearing Questions - Noobie
 

Excellent gears, at a reasonable price. USA made too. Just put the front and rear in my 69.

http://www.usgear.com/aftermarket.php

Re: ReGearing Questions - Noobie
 

Wow, Now I get that completely. I wont be towing anything with my truck, got the suburban for all that haha. But yes from reading all that, I'd agree 4.11s sound like the gearing set up for me. Now where would you suggest I get a set of gears from?

Re: ReGearing Questions - Noobie
 

Is this what I want? http://www.summitracing.com/parts/yga-24034/overview/

Re: ReGearing Questions - Noobie
 

Just a thought, but if you are going to regear both axles and buy new tires, you might look into swapping in some 3/4 ton or 1 ton axles. Add up the costs. I've done both. You're just going to be into your 10 bolts for some pretty decent coin. With 4.11 @ 55 mph and 35" tires you'll be at just over 1500 rpm in overdrive. 4.56 would put you at just under 1700. That 700r4 has a nice low first and second gear and I think you'd be happy with either. Remember you need installation kits too!

Re: ReGearing Questions - Noobie
 

Your right, I mean right now in overdrive while going 55mph I'm at 2000-2100k tons with 33s I personally like the amount of power

Re: ReGearing Questions - Noobie
 

Are you sure you have 3.08s?

Re: ReGearing Questions - Noobie
 

I mean 100% ? No. That's just what's in my dash, the dactory options sheet. I know for a fact the truck can hit 95 with 33s at 3k rpms I knows this because of the speeding ticket I got last year hah. Idk if this helps but if I'm at dead stop on flat ground, I can put the pedal to the floor (no brake) and spin my 33s no problem. The engine and trans have 365k on them too. It's crazy. I'm getting 10mpg or so too. But that's due to the carb, one it's a 350 carb and two I haven't had a chance to rebuild it yet.

Re: ReGearing Questions - Noobie
 

Sounds like you have 3.73 gears. I have 3.08, and Im at 2000@55 with no overdrive! On 33's. There are thick gears ( like the ones in your link), and thin ones, depending on original ratio. Different spline axles too. Make sure of what you have before buying.

Re: ReGearing Questions - Noobie
 

The axle shop that I buy my parts from, installs gears for $200- $250, plus parts. Cheap insurance of everything being right. If you have never done them before, you may want to consider having them done.

Re: ReGearing Questions - Noobie
 

And a new setup pinion bearing, dial indicator, calipers, and a depth checking tool.

Re: ReGearing Questions - Noobie
 

Ill have to take my rear apart at somepointbabd check that out for sure. I just got home and while driving Down the highway at 50mph (it's raining) I was at 1750 rpms in drive idk if that helps

Re: ReGearing Questions - Noobie
 

Yes, pull the cover, and count the teeth. Divide the two, and it will tell you the ratio. It may also say it on the ring gear.

Re: ReGearing Questions - Noobie
 

Alright I'll do that an get back to you this weekend.

Re: ReGearing Questions - Noobie
 

Just seeing this now, sorry bout that hah. But its funny im at the same rpm in 3rd, wonder if i do have 3.73s......

Re: ReGearing Questions - Noobie
 

Read this post here, it has some gearing infos....

http://67-72chevytrucks.com/vboard/s...d.php?t=568310

Re: ReGearing Questions - Noobie
 

Thanks for the link

Re: ReGearing Questions - Noobie
 

xx

Re: ReGearing Questions - Noobie
 

With 35" tires, your truck isn't going to get good fuel economy, no matter what you do. 35's are pretty big for a 10 bolt equipped truck anyhow. If you're just driving on the street, not a big deal. Going off road, I see snapped axle shafts in your future.

If you stick with your stock setup, 3.73 or 4.10 would be a good gear setup for 35"s.

If you go to a 700R4, I would seriously think about 4.56 gears. I think 4.10 would put alot of unneeded strain on the 700R4 when cruising down the highway at 60

Here's a gear calculator to play with

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

If you're going with lower gears, why not just swap to axles from a 3/4 ton or 1 ton truck? Most of them are already in the 3.73-4.56 range and it would solve any strength issues.

Re: ReGearing Questions - Noobie
 

3/4 or 1 ton axle assemblies are a lot heavier than 1/2 ton axle assemblies. That will also affect acceleration and mileage negatively. How do you intend to use your truck? Also, what kind of a budget are you on?

Re: ReGearing Questions - Noobie
 

It's true that the bigger axles are heavier and take a little more power to turn, but so is the tire you're putting on them. When you consider that these are c-clip axles, you're playing with fire regardless if you're on the street or 4 wheeling. Take a look at the Interco website. The differece between a 33 and 35 can be as much as 20 lbs different, depending on application. Now consider this, the 35 can be as much as 50lbs heavier than the stock rubber that came on these trucks. That's alot of rotating mass.

http://www.intercotire.com/tires.php?id=9&g=1

At the very least, a 14 B semi floater in the rear would be some added insurance. It fills the gap between the stock 10 bolt and the Full float axles without being too much.

Re: ReGearing Questions - Noobie
 

Well its been a while since ive posted this thread, Since then ive got mickey thompson classic 15x10 wheels and i love my 33s hah, So I'm still whiling to regear, I do have a 700r4 trans. I dont tow with it and most likely wont be. Thank you for the detailed response!

Re: ReGearing Questions - Noobie
 

Hah, tbh id love some dana 60s. The truck is my daily driver and will be for years to come. and my budget... uhhh hah its a typical 18 year olds but id say 700 or so

Re: ReGearing Questions - Noobie
 

Interesting. and the TSL radial in 33s on weighs 5 more pounds a piece than my bfgs ats, but tell me more bout the 14 bolt semi floater, like how does that exactly work and whats gunna run me?

Re: ReGearing Questions - Noobie
 

It has a 9.5" ring gear and heavier axle shafts than the 10 bolt.

GM started using in 3/4 ton trucks in 79. They used a 6 lug version of it starting with the 88-93 C2500 trucks. Some 3/4 ton six lug and 8 lug vans used it too. Some of the heavy 1/2 tons got them too. If I'm not mistaken, they're still using a version of it.


http://www.fourwheeler.com/how-to/tr...rs-and-locker/

http://www.gmt400.com/forum/showthre...-9-5-ring-gear

axle ID:
http://www.nationwideparts.com/differentials.htm

http://www.drivetrainamerica.com/t-sdl_technical.aspx

If you find an 8 lug version, you can always swap to the 6 lug axle shafts

http://www.moserengineering.com/cust...ine-axles.html

http://www.4wheelparts.com/Drivetrai...9.5in.+14+Bolt