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Old 04-12-2017, 08:07 AM   #1
albertan82
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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


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Istanbul, Turkey
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Old 04-12-2017, 08:40 AM   #2
James the III
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Re: Advice on the gear ratio for 350 vortec with TH350 -

Quote:
Originally Posted by albertan82 View Post
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
I'd get another tach and see if that one is reading correctly..

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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Old 04-12-2017, 08:44 AM   #3
cdub2010
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Re: Advice on the gear ratio for 350 vortec with TH350 -

http://www.4lo.com/calc/gearratio.php

plug in what numbers you want
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Old 04-12-2017, 08:46 AM   #4
James the III
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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
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Old 04-12-2017, 11:25 AM   #5
kwmech
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Re: Advice on the gear ratio for 350 vortec with TH350 -

Go with a 3.08
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Old 04-12-2017, 05:50 PM   #6
Dead Parrot
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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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Old 04-12-2017, 05:58 PM   #7
68gmsee
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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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Old 04-12-2017, 06:30 PM   #8
Alex V.
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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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Old 04-13-2017, 01:07 PM   #9
Tom
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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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