What is the best rear-end to run behind my 350?

[ubtripn]

What is the best rear end to run behind my 350 if I install a 700r4 and am looking for the best mileage out of my 68? Any input will be greatly appreciated. Thanks. I know nothing about the gear ratios.

[oldblue1968chevy]

373 seems like it'd be good with the OD trans

what motor/trans did your truck originally have?

[Longhorn Man]

you want to know the best gear, or the best rear end?

[72freak]

Its going to depend on what you are doing with the truck.

[Jeffs70]

Originally Posted by 72freak

Its going to depend on what you are doing with the truck.

like me I have a 373 posi 12 bolt rear end and I get around 12 mpg with that if thats what you just want. and it will pull me just fine from 0-60 mph in around 5 seconds even in this heavy truck.

[ubtripn]

Forgive my lack of knowledge. It will be a daily driver, not much towing, I want limited slip with gearing that will get me the best mileage. Truck had I believe a 327 with a Powerglide. Powerglide still in it.

[67ChevyRedneck]

3.73

[Longhorn Man]

if you live in flat lands, then a 3.73 will probably be best. If in the mountains, maybe something a little steeper, 4.11 maybe. The overdrive will knock the RPMs down quite a bit.

[Rooster's 67]

3.42

[BIGBLU 72]

3.73s! had that setup in my 97 tahoe! it was amazing i really miss that truck bc it was great on the highway, great around town and great tugging and pulling

[mr48chev]

Well, you would have to start with what you have now and go from there. Several of the guys will pipe in and say 4.11 gears but I'd go with 7.73 with the overdrive trans. That would let you cruise at highway speeds at a comfortable engine speed and still have a bit of zip off the line.

[N2TRUX]

Originally Posted by Rooster's 67

3.42

Correct. Guys he is looking for MPG not best performance. With a 350 and 700r4, a 3.42 is the best overall gear. If you want a bit more torque, a 3.73 is good but it will cost you 1-2 MPG.

[bajaracing]

72/c10. I have a mild 350, and 700r4 tranny. stock rear with 3.73 Detroit Truetrac Diff. Daily driver for me. No regrets here..

[Rooster's 67]

http://www.apexgarage.com/tech/gear_ratios.shtml

Using a 30" tall tire @ 2000RPM with stock 700r4 gearing and a final drive ratio of 3.42 you will have a cruise speed of 75MPH

1875 RPM at 70MPH w 3.42final

[72freak]

373 or the 342. If it was me I would try the 342 first.
I had an 89 throttle body set up in front of a 700R4 with 373's in my 1972 Suburban 2x4, 3/4 ton and averaged 12 in town and 15 on the highway. With a carb you wont do quite that good.

Keep in mind you wont get "good" gas mileage...just better gas mileage!

[Longhorn Man]

And if there is any kind of a hill at all, the trans will be kicking back and forth from 3rd to 4th. 2100 RPM is where you want to keep it. 18xx will start to lug it and hurt MPG

Originally Posted by Rooster's 67

http://www.apexgarage.com/tech/gear_ratios.shtml

Using a 30" tall tire @ 2000RPM with stock 700r4 gearing and a final drive ratio of 3.42 you will have a cruise speed of 75MPH

1875 RPM at 70MPH w 3.42final

[ubtripn]

All I can say is WOW! and thank you. You all know so much that I don't have to learn things the hard way wasting money. Thanks again, for my application it sounds like 3.73. When you say final drive ratio does that mean the tranny and rear end combined? Are all of the automatic 700R4's geared the same inside? (Again, novice in gearing) BigBlue and Bajaracing - what kind of mileage do you get? What is your highway cruising speed? Thanks again for all of the info!

Someone on this board needs to start a thread - who and how did people build, work on and find parts for their trucks before the web and this message board?

(Thanks for the correct on fuel mileage, it will be better, good is a judgment call.)

[cam.man67]

Yes, final drive technically is the rear axle ratio multiplied by the top transmission gear ratio. With a 3.42:1 rear end, your final drive is 2.394:1, given that a 700r4's top ratio is .70:1 .

[ubtripn]

Man thanks, this is like being in school but listening.

[vin63]

Originally Posted by ubtripn

Man thanks, this is like being in school but listening.

What's kind of missing in all of this is how tall (diameter) of a tire are you running or plan to run?

[Rooster's 67]

That's why I postulated a 30" tire the link in my previous post will calculate all you need to know and yeah you'll want to be up around 2100-2400 at cruise, below 2100 or so the converter may not have reached stall speed, causing increased trans fluid temps, lock up converter will help that but may hunt for lock up(lock in and out burning up the lock up in the converter)

Tire size will make a difference that is a variable you NEED to know to make an accurate choice
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[brad_man_72]

my 4l60e and 373 setup really likes a tall tire. I put some short tires on last year and hated them. I'd go 342 and fine tune w tire size.

[ubtripn]

They will be a taller, I don't like the big rim, little rubber look much. 30 inches is close.

[camaney]

Originally Posted by mr48chev

Well, you would have to start with what you have now and go from there. Several of the guys will pipe in and say 4.11 gears but I'd go with 7.73 with the overdrive trans. That would let you cruise at highway speeds at a comfortable engine speed and still have a bit of zip off the line.

7.73? wow thats gonna hurt mpg a lot. dont you think?

[FL71C10]

0 - 60 in 5 seconds? That seems a little optimistic.

Originally Posted by Jeffs70

like me I have a 373 posi 12 bolt rear end and I get around 12 mpg with that if thats what you just want. and it will pull me just fine from 0-60 mph in around 5 seconds even in this heavy truck.