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 > 47 - Current classic GM Trucks > The 1967 - 1972 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 06-11-2019, 06:44 PM   #1
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Rear end ratio help?

Can anyone give me an idea of my rear end ratio? I have a 69 longbed 350, 700r4 with a 12 bolt POSI rear end. When I’m doing 70mph my rpm is approx 2000rpm in 4 th gear ( lockup not used). oh I have 22 inch wheels on. I was thinking gearing feels a bit long and was wondering if it would be a bit more fun with a different rear end as I have play in the prop shaft anyway.

Last edited by Sparky dave; 06-12-2019 at 04:19 AM.
Sparky dave is online now   Reply With Quote
Old 06-11-2019, 07:38 PM   #2
Sheepdip
Registered User
 
Sheepdip's Avatar
 
Join Date: Nov 2018
Location: Waterford California
Posts: 209
Re: Rear end ratio help?

You'll need your rear tire actual O.D. not the rim size. Just punch in your rpm 2200, speed 70 and tire O.D. and hit calculate...... will then give you a close ratio.

https://speedmaster79.com/tools/gear-ratio/
Sheepdip is offline   Reply With Quote
Old 06-11-2019, 08:37 PM   #3
Mike C
Registered User
 
Mike C's Avatar
 
Join Date: Dec 1999
Location: Austin, TX, USA
Posts: 7,068
Re: Rear end ratio help?

22 inch wheels don't help at all. You need to provide the outside diameter of the wheel/tire combo. I would guess you had 3.07 and tires were 30" tall or so.
__________________
44 Willys MB
52 M38A1
64 Corvette Coupe
68 Camaro 'vert LT1 & TH700
69 Z/28 355 12.6's @110
69 Chevy Short Step 4 1/2"/7" drop
72 Jimmy 4WD 4spd 4" & 35's
02 GMC 2500HD 4x4 Duramax
Mike C is offline   Reply With Quote
Old 06-11-2019, 08:53 PM   #4
RustyBucket
Registered User
 
RustyBucket's Avatar
 
Join Date: Jul 2009
Location: Space Coast, Fl.
Posts: 887
Re: Rear end ratio help?

If you're in 4th gear with your 700r you're IN overdrive. Third gear is a 1 to 1 ratio. I think the O.D. ratio is approx. .70
__________________
Dad always said, "Son, WISH IN ONE HAND, and ......."
--------------------------------------
--------------------------------------

'71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C
'72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt
'66 Plymouth Belvedere City of Miami cop car clone

'70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!!
'67 C10 short/step side mount spare -- shop ornament, needs resto.... SOLD!!
'72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!!
'69 Nova 350, 4spd, A/C, ps, p.b, needs resto...SOLD!!
RustyBucket is offline   Reply With Quote
Old 06-12-2019, 04:10 AM   #5
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by Sheepdip View Post
You'll need your rear tire actual O.D. not the rim size. Just punch in your rpm 2200, speed 70 and tire O.D. and hit calculate...... will then give you a close ratio.

https://speedmaster79.com/tools/gear-ratio/
Thank you Iíll go work it out now. I did a diff oil change recently and never checked what rear end I had and donít fancy draining the oil again so this helps me a lot
Sparky dave is online now   Reply With Quote
Old 06-12-2019, 04:12 AM   #6
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by RustyBucket View Post
If you're in 4th gear with your 700r you're IN overdrive. Third gear is a 1 to 1 ratio. I think the O.D. ratio is approx. .70
Thanks for clearing that up. Ah so itís my converter lock up that Iím not using then. Will my rpm drop even more if I start using it
Sparky dave is online now   Reply With Quote
Old 06-12-2019, 04:14 AM   #7
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by Mike C View Post
22 inch wheels don't help at all. You need to provide the outside diameter of the wheel/tire combo. I would guess you had 3.07 and tires were 30" tall or so.
Ok mike Iíll get the tape measure out. Thanks
Sparky dave is online now   Reply With Quote
Old 06-12-2019, 09:31 AM   #8
rpmerf
Registered User
 
rpmerf's Avatar
 
Join Date: Feb 2011
Location: Parkville, MD
Posts: 657
Re: Rear end ratio help?

If you just give us the size, we can calculate the diameter. Don't need to get the tape out.
__________________
1970 C20 Custom Camper - 350, TH350
1997 GMC Suburban
1994 Acura Integra GSR
1987 Dodge Daytona Shelby Z
rpmerf is offline   Reply With Quote
Old 06-12-2019, 09:41 AM   #9
rpmerf
Registered User
 
rpmerf's Avatar
 
Join Date: Feb 2011
Location: Parkville, MD
Posts: 657
Re: Rear end ratio help?

Given that you are at 2000 RPM at 70 MPH, that means you are doing 35 MPH per 1000 RPM. This is the ratio in my suburban with 245/75R16 (30.5" diameter) and 3.73 rear gears. If your tires are smaller, you have a lower ratio rear, if your tires are larger, you have a higher ratio rear.
__________________
1970 C20 Custom Camper - 350, TH350
1997 GMC Suburban
1994 Acura Integra GSR
1987 Dodge Daytona Shelby Z
rpmerf is offline   Reply With Quote
Old 06-12-2019, 09:54 AM   #10
tdangle
Senior Member
 
tdangle's Avatar
 
Join Date: Mar 2005
Location: Jurupa Valley, Ca
Posts: 512
Re: Rear end ratio help?

Quote:
Originally Posted by rpmerf View Post
Given that you are at 2000 RPM at 70 MPH, that means you are doing 35 MPH per 1000 RPM. This is the ratio in my suburban with 245/75R16 (30.5" diameter) and 3.73 rear gears. If your tires are smaller, you have a lower ratio rear, if your tires are larger, you have a higher ratio rear.
that doesn't sound correct. with 3.73 and those tires with a th350 at 70 I would think you are closer to 3000 rpm. I have slightly taller tires 4.10 and .7 overdrive with lockup and at 70 I'm right at 2000
__________________
Terry

1970 C20 Custom Camper, 350/2004R, Dana 60, factory AC, side tool box
tdangle is offline   Reply With Quote
Old 06-12-2019, 11:30 AM   #11
rpmerf
Registered User
 
rpmerf's Avatar
 
Join Date: Feb 2011
Location: Parkville, MD
Posts: 657
Re: Rear end ratio help?

My suburban is a 97 - overdrive with lockup. I can plug it into a calculator and see 34.7 MPH per 1000 RPM.

To get ~35 MPH / 1000 RPM with a 0.7:1 gear ratio and a 4.10 rear, you would need to have ~33.5" tires.

I'm assuming you have 235/85R16, as that's what came on my 70 C20. They are 31.7". With OD and a 4.10 rear that should give 32.9 MPH / 1000 RPM. You should be closer to 2,125 RPM at 70 MPH.

Here is a calculator to get the speed at a given tire size, trans ratio, rear ratio, and RPM.
https://spicerparts.com/calculators/...rpm-calculator
__________________
1970 C20 Custom Camper - 350, TH350
1997 GMC Suburban
1994 Acura Integra GSR
1987 Dodge Daytona Shelby Z
rpmerf is offline   Reply With Quote
Old 06-12-2019, 01:18 PM   #12
tdangle
Senior Member
 
tdangle's Avatar
 
Join Date: Mar 2005
Location: Jurupa Valley, Ca
Posts: 512
Re: Rear end ratio help?

Quote:
Originally Posted by rpmerf View Post
My suburban is a 97 - overdrive with lockup. I can plug it into a calculator and see 34.7 MPH per 1000 RPM.

To get ~35 MPH / 1000 RPM with a 0.7:1 gear ratio and a 4.10 rear, you would need to have ~33.5" tires.

I'm assuming you have 235/85R16, as that's what came on my 70 C20. They are 31.7". With OD and a 4.10 rear that should give 32.9 MPH / 1000 RPM. You should be closer to 2,125 RPM at 70 MPH.

Here is a calculator to get the speed at a given tire size, trans ratio, rear ratio, and RPM.
https://spicerparts.com/calculators/...rpm-calculator
Sounds about right. didn't realize you were comparing to a different vehicle than in your signature. And yes I have 265-75-16 with a dia of about 31.6" so just over 2K is correct. Way nicer than when I had the TH350 trans.
__________________
Terry

1970 C20 Custom Camper, 350/2004R, Dana 60, factory AC, side tool box
tdangle is offline   Reply With Quote
Old 06-13-2019, 01:03 AM   #13
Overdriven
Registered User
 
Join Date: Mar 2016
Location: Long Island NY
Posts: 493
Re: Rear end ratio help?

Quote:
Originally Posted by Sparky dave View Post
Thanks for clearing that up. Ah so itís my converter lock up that Iím not using then. Will my rpm drop even more if I start using it
Yes, rpm will drop more as torque converter slip is eliminated with lockup engaged. Typical rpm drop is around 300rpm but depends on the converter.
Overdriven is offline   Reply With Quote
Old 06-13-2019, 01:07 AM   #14
randy500
Senior Member
 
Join Date: Jul 2016
Location: Tacoma, WA
Posts: 5,273
Re: Rear end ratio help?

You probably won’t have to worry about the gear ratio much longer, trans will be blown before you know it without lockup in overdrive....
randy500 is offline   Reply With Quote
Old Today, 06:37 PM   #15
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by randy500 View Post
You probably wonít have to worry about the gear ratio much longer, trans will be blown before you know it without lockup in overdrive....
Is lockup for all gears or just 4th? Iím hardly ever in 4 th long enough to need it but if itís real important I best wire it in then
Sparky dave is online now   Reply With Quote
Old Today, 06:38 PM   #16
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by Overdriven View Post
Yes, rpm will drop more as torque converter slip is eliminated with lockup engaged. Typical rpm drop is around 300rpm but depends on the converter.
Thanks for your help mate appreciate it
Sparky dave is online now   Reply With Quote
Old Today, 06:47 PM   #17
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Tyre size is 285 35 22
Sparky dave is online now   Reply With Quote
Old Today, 06:52 PM   #18
Sparky dave
Registered User
 
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 254
Re: Rear end ratio help?

Quote:
Originally Posted by rpmerf View Post
Given that you are at 2000 RPM at 70 MPH, that means you are doing 35 MPH per 1000 RPM. This is the ratio in my suburban with 245/75R16 (30.5" diameter) and 3.73 rear gears. If your tires are smaller, you have a lower ratio rear, if your tires are larger, you have a higher ratio rear.
It comes out at 29.9 in tyre diameter so Iím thinking it might be 3.73 s like yours. Thanks for the info
Sparky dave is online now   Reply With Quote
Reply

Bookmarks

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 07:24 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.
Copyright 1997-2013 67-72chevytrucks.com