The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Engine & Drivetrain

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 04-12-2017, 08:07 AM   #1
albertan82
Registered User
 
Join Date: Sep 2015
Location: Istanbul
Posts: 5
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
albertan82 is offline   Reply With Quote
Old 04-12-2017, 08:40 AM   #2
James the III
Registered User
 
Join Date: Nov 2016
Location: lowell ma
Posts: 750
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.
James the III is offline   Reply With Quote
Old 04-12-2017, 08:44 AM   #3
cdub2010
Senior Member
 
Join Date: Apr 2014
Location: detroit mi
Posts: 205
Re: Advice on the gear ratio for 350 vortec with TH350 -

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

plug in what numbers you want
cdub2010 is offline   Reply With Quote
Old 04-12-2017, 08:46 AM   #4
James the III
Registered User
 
Join Date: Nov 2016
Location: lowell ma
Posts: 750
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
James the III is offline   Reply With Quote
Old 04-12-2017, 11:25 AM   #5
kwmech
Senior Member
 
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,355
Re: Advice on the gear ratio for 350 vortec with TH350 -

Go with a 3.08
kwmech is offline   Reply With Quote
Old 04-12-2017, 05:50 PM   #6
Dead Parrot
Registered User
 
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,461
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.
Dead Parrot is offline   Reply With Quote
Old 04-12-2017, 05:58 PM   #7
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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.
__________________
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...
68gmsee is offline   Reply With Quote
Old 04-12-2017, 06:30 PM   #8
Alex V.
Registered User
 
Alex V.'s Avatar
 
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
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.
__________________
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.
Alex V. is offline   Reply With Quote
Old 04-13-2017, 01:07 PM   #9
Tom
driving is in my blood
 
Tom's Avatar
 
Join Date: Dec 1999
Location: Mesa AZ
Posts: 5,687
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.
__________________
-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.
Tom is offline   Reply With Quote
Reply

Bookmarks

Tags
differential, drivetrain, gear ratio, th350, vortec

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 09:08 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com