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04-12-2017, 08:07 AM | #1 |
Registered User
Join Date: Sep 2015
Location: Istanbul
Posts: 5
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Advice on the gear ratio for 350 vortec with TH350 -
Hi ,
I have a 1964 C10 Stepside - short wheelbase, with 350 5.7 vortec and TH350. Tires are 225/75R15 (28.5"). Current differential is from a 1973 Chevrolet Caprice (350 engine) added to the truck by the previous owner, but I do not know the current gear ratio. It is the 10 bolt, 8.5", 28 spline differential. When I drive on the highway at 60mph, tach is showing around 4000rpm ( What gear ratio would be the best for this engine-trans-tire combo? Thank you all in advance From: Istanbul, Turkey |
04-12-2017, 08:40 AM | #2 | |
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Join Date: Nov 2016
Location: lowell ma
Posts: 750
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Re: Advice on the gear ratio for 350 vortec with TH350 -
Quote:
at 4000 rpm with a 28" tall tire, and a 1 to 1 3rd gear (th350) at 60 mph is a rear gear of 5.56 not likely.. you can get a ballpark,, by putting the rear axle of the car on jack stands.. mark the driveshaft. and mark the tire put the car in neutral , car not running.. and turn the driveshaft until the tire makes one full turn/rotation. counting the # of times the drive shaft turned.. it turns 3 and 3/4 times.. most likely have 3.73 gears 3 and 1/5th 3.23 just over 3 3.07 3 and just under a 1/2 3.42 little over 4 times. 4.11 etc. |
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04-12-2017, 08:44 AM | #3 |
Senior Member
Join Date: Apr 2014
Location: detroit mi
Posts: 209
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Re: Advice on the gear ratio for 350 vortec with TH350 -
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04-12-2017, 08:46 AM | #4 |
Registered User
Join Date: Nov 2016
Location: lowell ma
Posts: 750
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Re: Advice on the gear ratio for 350 vortec with TH350 -
http://www.wallaceracing.com/calc-rgr.php
Calculate Rear Gear Ratio for given MPH, RPM and Trans Gear Ratio When I plug in the numbers but in the trans gear ratio I put the th350 2nd gear ratio of 1.52 I get a more normal 3.66 and with the speedo being off, your tach being a tad off.. and me using 28" tire not 28.5" it get close to a g.m. 3.42 rear gear.. So.. let me ask, are you sure it's going into high gear.. and not going down the highway in 2nd |
04-12-2017, 11:25 AM | #5 |
Senior Member
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,638
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Re: Advice on the gear ratio for 350 vortec with TH350 -
Go with a 3.08
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04-12-2017, 05:50 PM | #6 |
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Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,613
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Re: Advice on the gear ratio for 350 vortec with TH350 -
Since it is a mix of parts, it is very likely that the speedometer gear unit in the transmission isn't correct for your setup and giving a wrong speed reading. Get a chase vehicle or gps unit and confirm your speed.
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04-12-2017, 05:58 PM | #7 |
Active Member
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: Advice on the gear ratio for 350 vortec with TH350 -
Agree with a 3.08. That's what I have on my 69 and it was a decent street ratio and also had no trouble pulling an 18' trailer with a full size tractor and shredder (no tall hills).
As for the ratio on your truck, in some cases there's a number stamped on top of the passenger side axle tube. Years of grime may have covered it so use a wire brush to clean it if it's crudded up. I found removing the passenger side tire gave me a better view of the number on mine.
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68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive... |
04-12-2017, 06:30 PM | #8 |
Registered User
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
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Re: Advice on the gear ratio for 350 vortec with TH350 -
Stock engine? 3.08's if you don't want it to pull very much comfortably, around a 3.36 or maybe a 3.42 (popular in the 90's 1/2 ton 2WD trucks) if you want it to tow a little better.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
04-13-2017, 01:07 PM | #9 |
driving is in my blood
Join Date: Dec 1999
Location: Mesa AZ
Posts: 5,748
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Re: Advice on the gear ratio for 350 vortec with TH350 -
I wouldn't do anything steeper then 3.07's, you don't have overdrive. You'll still be at 3k running 80mph with 3.07's.
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-78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me. |
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differential, drivetrain, gear ratio, th350, vortec |
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