ReGearing Questions - Noobie

[SnowNeck]

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!

[dieseldawg142]

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

[SnowNeck]

Originally Posted by dieseldawg142

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

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

['63GENIII]

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.

[SnowNeck]

Originally Posted by '63GENIII

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.

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?

[SnowNeck]

Originally Posted by dieseldawg142

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

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

[dieseldawg142]

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

[willett]

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

[argonaut]

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

[SnowNeck]

Originally Posted by argonaut

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

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

[SnowNeck]

Originally Posted by argonaut

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

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

[MTCK]

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.

[SnowNeck]

Originally Posted by MTCK

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.

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?

[argonaut]

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?

[SnowNeck]

Originally Posted by argonaut

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?

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.

[argonaut]

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

[argonaut]

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.

[SnowNeck]

Originally Posted by argonaut

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.

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?

[Dunenutt]

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

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

[MTCK]

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!

[SnowNeck]

Originally Posted by MTCK

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!

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

[Dunenutt]

Originally Posted by MTCK

Remember you need installation kits too!

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

[argonaut]

Are you sure you have 3.08s?

[SnowNeck]

Originally Posted by argonaut

Are you sure you have 3.08s?

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.

[Dunenutt]

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.