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Old 04-20-2011, 10:19 AM
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Rebuilding a diff - what ratio to use with my T-350?

Hey everyone, I tried doing a search on this, but I got too many variables back. If there is a thread for this already, please let me know!

What I'm wondering is how to choose a gear ratio based on the transmission I have (TH350C). I'd also like to compare this finding to what a 700R might do for me.

I'm, going to be rebuilding a 10 bolt (8-1/2") GM posi and want to go with the highest ratio I can get away with without highway RPM being a huge killer.
I'd like to keep it under say 2200 rpm at 60MPH (100KMH). I'll also be using the 15" factory rally rims with 65 series rubber.

Would it be possible to get away with 3.42's or 3.73's using a 700R trans?

thanks in advance.
Again, if there is an existing thread, please direct me there

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Old 04-20-2011, 10:57 AM
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my 72 blazer that is shown here has a new gen 454--at the moment it still posts a 350 turbo--been working on upgrade , but all the other projects "WOW"
anyway i would sugest 342 to 350 in gears--i'm at 373,and 350 tubo w/ 295x50x15 (26 1/2") rpm is right about 2200 at 60--i'v already spent some time on this ,and if i went to 342 or 350 class it would drop r's to 1900-2000--the best process is what i have in my dulley--342 x 454 x 4 L 80===1900 at 60 mph w/ plenty of guts ! one other option is the late 70's to early 80's 350 w/ loc up torq conv. a switch on the dash , and one at break peddle , and you are good to go.......

my thoughts......steven
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Old 04-20-2011, 12:21 PM
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what kind of vehicle are we talking about?
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Old 04-20-2011, 02:30 PM
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72 blazer ----------my "avatar"
2 wheel dr.

Last edited by rusobr2; 04-20-2011 at 02:35 PM. Reason: delete message
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Old 04-20-2011, 02:53 PM
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Having a 305 performer build vortec 85 gm truck , with a 350C Lockup on a B+M unit, the 3.73 gears work right well with a 28" tire. The rpm is at 2850 at 65mph. Any more and forget pulling on the interstate, any less and the truck feels as if held back.

Say 17 mpg , just at one held speed in the slow lane, cruise control on at 65 mph. Any faster is wastng fuel cause the frontal area acts like a air brake.

Tows well, burns the tires out the hole, runs 15.7 at 87mph. It burys the speedo in 15 sec.
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Old 04-20-2011, 03:00 PM
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Quote:
Originally Posted by rusobr2
72 blazer ----------my "avatar"
2 wheel dr.
i was asking the original poster
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Old 04-20-2011, 04:26 PM
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Thanks guys. I didn't post the vehicle as I didn't think it mattered as far as calculations went. It's a late 80's GM pickup (2 wheel drive). The cruising speed of this truck will be around the 45-60 mph range (80 to 100kmh). Most of it's use will be for short trips into town, towing the occaisonal load and some mild abuse of the loud pedal.

I like the numbers that Rusobr2 is posting, but I don't want the revs to get up as high as what Spinn is showing.

The only real difference between the two seems to be the overall wheel diameter - does an inch and a half difference between tire heights really make that much difference?
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Old 04-20-2011, 04:49 PM
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with a 700r4 and 3.73 gears at 62 mph (100km/h)
26" tall tires 2100 rpm
26.5" 2050 rpm
27" 2015 rpm
27.5 1980 rpm
28" 1940 rpm
28.5 1910 rpm

4:10
26" 2300 rpm
26.5" 2250 rpm
27" 2210 rpm
27.5" 2175 rpm
28" 2135 rpm
28.5" 2100 rpm
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Old 04-20-2011, 05:56 PM
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I run 30" tires on my T-Bucket with 3.73 gears. I turn 2500 rpm at about 60 mph. I find myself wishing the rpms were a little lower when on long highway drives. An overdrive trans in place of my TH350 has crossed my mind a number of times. Feels good for the very few times when I really get on it, but I originally had a 57 Chev diff with a 3.31 gear set in it and it was much better on the highway. If doing it again I probably would stick with the 3.31.
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Old 04-21-2011, 05:55 PM
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Excellent, thanks for the list of rpms vs. tire diameter VA.
cuke, 2500 would be too much for me too.
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