rearend gears

[bigyellow]

i've made a couple posts on here asking for opinions, nso now i'm curious as to what rearend gears i should run with my t5 that has a .72 5th gear.

i have two rearends for my truck, one is a 3.73 and the other is a 3.08. nothing is wrong with either one of them but i've read that 250 straight sixes don't like to pull freeway speeds with a OD trans and a gear like the 3.08.

i don't get on the freeway often but i probably would if i could get on it and not get passed by every single little civic because i can't go over 63 without watching the fuel hand drop rapidly.

so any opinions on what rearend to use would be great =)

[Captainfab]

Tire size also plays into the equation, but I would go with the 3.73 gear versus the 3.08.

[bigyellow]

i run a 275-60-15.

is it really that bad to drive with the 3.08?

[crazy longhorn]

The 3.08 will lug your inline too hard in OD, & you would most likely loose mileage. My vote also for the 3.73 gears I run a 3.42 ,with a .76 OD, & its a cruiser....but that 383 has a lot of tork! crazyL

[Wrenchbender Ret]

Your o-drive will be worthless with a 3:08 & a 250.

[crazy longhorn]

You need to understand the "sweet spot" on that,or any engine. Run it too slow, & you loose mileage....run it too fast,& you loose mileage. You wont get hy speed mileage out of these old(or new trucks). You are tryin to shove a brick through the wind @ speedmy old silverado gets better mileage running 70-72mph, than it does @ 55. After that , the mileage goes down! All the gear in the world cant help that, its like shovin a brick through the wind! take your old toy out & play a bit....find the sweet spot on the gear you have, then figure how fast you like to cruise, & figure the gear from that spot. both of my trucks play this way....the 02 silverado ,has a 4.8 , & the 69 is running a 383.....somewhere around 2200 hits the "sweet spot", on both trucks in OD. The old 69, puts that sweet spot @ a little higher speed(dropped,4" chopped top, & more tork).....basically less "brick" goin through the wind....I hope that helps a bit, crazyL

[68Gold/white]

Originally Posted by crazy longhorn

You need to understand the "sweet spot" on that,or any engine. Run it too slow, & you loose mileage....run it too fast,& you loose mileage. You wont get hy speed mileage out of these old(or new trucks). You are tryin to shove a brick through the wind @ speedmy old silverado gets better mileage running 70-72mph, than it does @ 55. After that , the mileage goes down! All the gear in the world cant help that, its like shovin a brick through the wind! take your old toy out & play a bit....find the sweet spot on the gear you have, then figure how fast you like to cruise, & figure the gear from that spot. both of my trucks play this way....the 02 silverado ,has a 4.8 , & the 69 is running a 383.....somewhere around 2200 hits the "sweet spot", on both trucks in OD. The old 69, puts that sweet spot @ a little higher speed(dropped,4" chopped top, & more tork).....basically less "brick" goin through the wind....I hope that helps a bit, crazyL

X2. You have to have enough torque on hand to easily run the vehicle at highway speed, even if you don't drive i on the hoghway that much. Gear it right and the highway AND cruising in town MPG will be better.
It's kinda like starting out riding your 10 speed bicycle in 10th gear, it can be done, but it's not easy...
Fuel effeciency w/ one of these pickups will never be great. You definately need the 3.73 gear...

[Longhorn Man]

I agree 100% with all said above. Not many people (regardless of engine) get good MPG in these trucks with an overdrive and 3.08 gears. It happens sometimes, but not as often as one would think.

[bigyellow]

i'm not going for pure gas mileage improvement mostly better driveability and practicality. i'm pretty happy with the mpgs i get now considering it is a brick with an old carbed motor. right now my sweet spot is about 63-67. thats where the engine purrs along without lugging it and not revving on it too hard. but it is a little too slow for the freeway lol. i mean i can't really say much i've got it up to about 90 before and that was screaming, especially without a sway bar. it was pretty scary. lol

[Longhorn Man]

I still recomend the 3.73 though... no reason to lug the engine if you don't need to.

[blackcherry81]

hey guys this is a little off the current subject but the thread is called rearend gears lol. i currently have a 5 lug 12 bolt rearend with 3;08 gears in my 81 c10 and im wating to put a unit with 373s in and i recently found a junkyard close to home that has tons of squarebodys blazers suburbans an the like and i was wondering if anyone knew what would be the best way to sort through the trucks as far as years, engine type, body type, or whatever to find the rearend i want.all answers will be much appreciated

[bigyellow]

if it were me i'd find rearend identification numbers and make a list of rearends that have the gears you want then just start crawling under some trucks until you find the one you want.

[karl]

Not sure about your setup. But i have a 350/350 with a mild rv cam, headers, quadrajet carb and 307 gears and i can easily get a 2nd gear scratch with 295/50/15s out back. And it sails on the freeway. Cant wait to swap in my hi perf 383. If you have some torque the 307s will be good. If you dont have alot of torque coming out of your engine and/or have a manual trans 4-5 speed i would use the 373s.

[Longhorn Man]

karl... he's talking about an overdrive with these gears... not a 3 speed like you have

[blackcherry81]

thanks for the advice guys, i now know that the rpo code for a 3;73 rear gear ratio in a 73-87 is (gt4) i guess that would be on the option sticker in the glovebox?? or is there another place to look for this code or way to identify a vehicle with this rearend that anyone knows of

[oldblue1968chevy]

best way is pull the cover, cant hurt to put 8 bucks worth of new oil in it anyways..

[VDOG]

Originally Posted by karl

Not sure about your setup. But i have a 350/350 with a mild rv cam, headers, quadrajet carb and 307 gears and i can easily get a 2nd gear scratch with 295/50/15s out back. And it sails on the freeway. Cant wait to swap in my hi perf 383.

Now thats the way I was going to go, 350/350 setup with the same parts. then I decided to go 383. My delima, I had my rearend rebuilt, 3:08's so can I install my OD trans with 3:08 gears and a 383, will this work? Or should I just stay with the 350 combo? Im not about to open the rear back up, it was 4 years ago, yet I dont have 1000 miles on it. I need a combo that will work.

[justahoby]

Originally Posted by VDOG

Now thats the way I was going to go, 350/350 setup with the same parts. then I decided to go 383. My delima, I had my rearend rebuilt, 3:08's so can I install my OD trans with 3:08 gears and a 383, will this work? Or should I just stay with the 350 combo? Im not about to open the rear back up, it was 4 years ago, yet I dont have 1000 miles on it. I need a combo that will work.

I run a 3.08 with an NV4500 on a 383, 2wd, i keep the mileage to myself as all will yell liar.. expect at least 20 if at only 55-60... 3 blade fan light duty fan clutch, and no dont run much bigger than an rv cam or 4x4 cam... And dont run an automatic... Also run narrow tires, keep my toe in in check, and need to put power steering belt on. Wicked mileage on flatlands, and tall initial timing with a adjustable advance. For me is fine, but if you live in the hilly mountain regions, 5th is quite less usefull. And a ton of tuning to get where im at.. FGot pulled over with a warning, didnt realize i was shy of triple digits. It will walk away on ya!

[edhtrd71]

I agree with everyone. Your getting good advice. 3.73s