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Old 05-09-2017, 08:14 PM   #1
chris mc bride
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3.73 or 3.42

Well Panel needs rear end rebuilt.Trying to decide between 3.73 and 3.42 gear. While it does have original 3 spd on the tree,it will be getting 4L80E in near future. Truck is mainly driver,cruiser and will see some lite towing.Tow wise my FED on open trailer for close races (100 miles and under) will be majority.Weight maybe 4000 if 4 wheelers loaded also.

Will I be giving up much pull power by going with 3.42. I know I loose little on fun meter. MPG wise is there really much difference once I get auto with OD installed?
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Old 05-09-2017, 08:19 PM   #2
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Re: 3.73 or 3.42

You don't mention the engine that you have but if you're going to use it mainly for cruising and light towing I'd go with the 3.42.

My 69 had a 350/350 with a 3.08 rear axle and it did fairly well on light towing. On a few occasions I towed over 4K lbs on a 16 foot trailer (flat country mostly) with no problems.
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Old 05-09-2017, 08:25 PM   #3
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Re: 3.73 or 3.42

Good point 454
Plans are for 049 heads with 2.25/1.88 valves 5 angle valve job and mild porting .Already working on these.
Edlebrock performer rpm plus cam and intake
2 headers with 2-1/2'' exhaust with crossover and magna flow
should cover all points.
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Old 05-09-2017, 08:29 PM   #4
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Re: 3.73 or 3.42

Take into account tire size, final gear of trans, choosen gear ratio and find the sweet spot for your engine...http://www.csgnetwork.com/multirpmcalc.html
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Old 05-09-2017, 10:45 PM   #5
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Re: 3.73 or 3.42

Already been to Wallace calculator site. Difference with trans swap is not big deal. With 3 spd its pretty good difference.I think 3.73s at 70 mph is around 3100 and with 342's its still 2800. 25% reduction is big.
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Old 05-10-2017, 02:52 AM   #6
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Re: 3.73 or 3.42

I would probably go with 3.73 if it were mine and given those 2 choices..mine has 4.10 with o.d. and I love it but I went from s.b. to LS..and im pulling around 7-9000 lb loads.
I havnt looked but what's the rpm range on your cam and intake choice
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Old 05-10-2017, 07:51 AM   #7
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Re: 3.73 or 3.42

I have a 4l60e with 3.73 posi, 29" tires. I like how responsive it is and yet I get 20mpg on the highway at @2100rpm.
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Old 05-10-2017, 09:23 AM   #8
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Re: 3.73 or 3.42

according to desk top dyno(normally real close) I should have 510 lbs. tq plus from 1500 thru 5000 with peak of about 550 at 4000 rpm. Provided I can make heads low as well as set I used number from.LOL
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Old 05-10-2017, 11:57 AM   #9
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Re: 3.73 or 3.42

I'd go with 3.73 and then the future trans swap.
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Old 05-10-2017, 06:26 PM   #10
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Re: 3.73 or 3.42

Looks like the 3.73s are going to win. While may cost me some till I get the 4L80E installed with stand alone. Maybe a little while since stumbled into deal on heads I was looking for. Now to explain to ol lady how I saved money while dropping 200 bucks on heads needing 300 to 400 bucks of work.LMAO Nothing new to her with race car in garage.
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Old 05-10-2017, 07:55 PM   #11
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Re: 3.73 or 3.42

3.73's+ 4l80's 4th gear =way too many rpm's for anything other then max rated tow load. My 350 with 3.42's can tow anything I can hitch up without issues, can't see the need for more rpms with a big block.
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Old 05-10-2017, 08:52 PM   #12
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Re: 3.73 or 3.42

According to calculator its about 192 rpm difference in 4th gear @ 70 2160 vs 2352.Maybe just me but that not a lot of difference. Now at 85 its almost 420 rpm. low would be roughly 8.53.:1 vs9.31 :1 ratio wise that's a lot of pull off or tire frying difference. Got to play some too. This big block does have better than stock valvetrain and bottom end.Drag racer can never do build w/o upgrades.LOL
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Old 05-10-2017, 11:12 PM   #13
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Re: 3.73 or 3.42

This somewhat depends on the rear tire size, but I would go with 3.42's. That is what I have in my Suburban with a TH400 and 275/60R15 tires. An OD gear would be nice for highway cruising with this rear gear and tire combo.
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Old 05-11-2017, 08:25 AM   #14
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Re: 3.73 or 3.42

3.42
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Old 05-11-2017, 09:27 AM   #15
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Re: 3.73 or 3.42

Quote:
Originally Posted by chris mc bride View Post
..... that's a lot of pull off or tire frying difference. Got to play some too. ...
You hadn't mentioned doing some tire smoking on the side but if you intend to do that on occasion, then the 3.73 would be a better selection.

If your intent is to save some on gas and still do some light towing than it's the 3.42.
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Old 05-11-2017, 10:17 AM   #16
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Re: 3.73 or 3.42

If you have some kind of limited slip/posi-trac as a possible future addition, check compatibility with both your ratio options. Some brands/models will work with one but not the other.
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Old 05-12-2017, 11:54 AM   #17
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Re: 3.73 or 3.42

First. Is smoking tires on occasion ever a question? LOL

Second I either will work with posi unit since they both fit same 3 series carrier. Yes is on future wish list. Then so are about $5000.00 of other small items thein few big ticket items.
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Old 05-15-2017, 10:24 PM   #18
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Re: 3.73 or 3.42

Go 3.73, life is short.
Chances are you will not see a huge difference in mileage with 3.42.
A warm up big block is not know for gas mileage!!
Have fun with your truck!
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