Rear end gear ratio for big block - CorvetteForum - Chevrolet Corvette Forum Discussion

Register FAQ Search Today's Posts Mark Forums Read Vendor Directory

C3 Tech/Performance
V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette

Rear end gear ratio for big block

Reply
 
 
 
Thread Tools Search this Thread
Old 06-14-2010, 10:24 AM   #1
Red_Shift
CF Senior Member
Thread Starter
 
Red_Shift's Avatar
 
Member Since: Jul 2004
Location: Lapeer MI
Posts: 420
Thanks: 0
Thanked 0 Times in 0 Posts
Default Rear end gear ratio for big block

After searching through several posts reguarding the rear gear ratios I'm a little confused as to what rear end gear I should use. The vital info follows:

1975 coupe
zz502 (5800 redline)
Turbo 400 trans (I believe 3rd gear is 1:1)
2800rpm stall converter
275/35 R18 tires (these are 25.57" diameter)
Tread width: 10.8"
I will drive this car on sunny weekends and at the drag strip on occasion.

So the zz502 puts out a lot of torque...

Would this suggest a lower gear like 3.08? I currently own two gears, 3.08 and 3.90. If I install the 3.90 then I'm looking at 3587 rpm @ 70mph (seems high) and will red-line at 113mph (based on my understanding the T400 is 1:1 in 3rd gear).

That 3.90 gear is too tall right?
What gear would you run?
Red_Shift is offline   Reply With Quote
Old 06-14-2010, 11:47 AM   #2
bashcraft
CF Senior Member
 
bashcraft's Avatar
 
Member Since: Jan 2006
Location: Butler Pa
Posts: 5,537
Thanks: 0
Thanked 17 Times in 10 Posts
Default

What gear is in it now?

I have the same motor as you but a 4 speed trans and will be replacing the current 3.55 rear with a 3.08
bashcraft is offline   Reply With Quote
Old 06-14-2010, 11:57 AM   #3
midyearvette
CF Senior Member
Support Corvetteforum!
 
midyearvette's Avatar
 
Member Since: Nov 2008
Location: columbus oh
Posts: 5,694
Thanks: 0
Thanked 1 Time in 1 Post
Default

336....jmo....
midyearvette is offline   Reply With Quote
Old 06-14-2010, 12:38 PM   #4
roscobbc
CF Senior Member
 
roscobbc's Avatar
 
Member Since: Oct 2005
Location: East London/SW Essex UK
Posts: 1,025
Thanked 1 Time in 1 Post
Default

I run with M21, 3.08 behind a 528hp/564 ftlb 489 - with 285/40/18's it cruises 70mph close to 2600 rpm - its fine off the line too
roscobbc is offline   Reply With Quote
Old 06-14-2010, 01:11 PM   #5
ajrothm
CF Senior Member
 
ajrothm's Avatar
 
Member Since: May 2006
Location: League City Tx
Posts: 7,291
Thanked 108 Times in 91 Posts
Default

You won't like the 3.90s...the stock zz cam is done by 5200-5400 rpms and with 3.90 gears, you will be out of steam before the end of the 1/4.

I would stick with 3.36s or 3.08s since you have them. The converter will do more for you then the gears anyway.

I have a 496 and have 3.08s. It's highway friendly and the BB torque combined with a good converter pulls them fine. That being said, when I rebuild the IRS I am going to 3.36s for a little better ETs and street grunt.
ajrothm is offline   Reply With Quote
Old 06-14-2010, 04:36 PM   #6
Red_Shift
CF Senior Member
Thread Starter
 
Red_Shift's Avatar
 
Member Since: Jul 2004
Location: Lapeer MI
Posts: 420
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Thanks AJ and thanks to others who have replied.

roscobbc, your tires are 1.5 inches taller than mine. I'm not sure what the final output ratio of the M21 is. Thanks for the input.

AJ, what transmission you running?
Red_Shift is offline   Reply With Quote
Old 06-14-2010, 04:57 PM   #7
ajrothm
CF Senior Member
 
ajrothm's Avatar
 
Member Since: May 2006
Location: League City Tx
Posts: 7,291
Thanked 108 Times in 91 Posts
Default

Quote:
Originally Posted by Red_Shift View Post
Thanks AJ and thanks to others who have replied.

roscobbc, your tires are 1.5 inches taller than mine. I'm not sure what the final output ratio of the M21 is. Thanks for the input.

AJ, what transmission you running?
I also have a T400, "tight" 10" converter.... At 65mph I am around 2750rpms, 70mph(gps) is 2950rpms. Converter seems to add about 200rpms or so at highway cruise. I like the 3.08s and 255/60s for highway cruising and my car still leaves pretty decent at the track (1.61 60') but if I am gonna spend the loot to rebuild the rear end and put all the goodies in there, I want to step it up just a notch....3.36 is as low as I'm going...some day when I hit the lotto and convert to a 200r4 I will be glad I up'd the gear some.

Just make sure you use a good converter, not a speed shop cheapy as they tend to slip way too much at all speeds and hurt trap speed..
ajrothm is offline   Reply With Quote
Old 06-14-2010, 05:09 PM   #8
Rich N.
CF Senior Member
 
Member Since: Sep 2006
Location: Upper Darby PA
Posts: 153
Thanks: 0
Thanked 0 Times in 0 Posts
Default

308 for sure.
Rich N. is offline   Reply With Quote
Go Back   CorvetteForum - Chevrolet Corvette Forum Discussion >
Reload this Page
  • Rear end gear ratio for big block
  •  
     
    Reply

    Related Topics
    Thread Thread Starter Forum Replies Last Post
    700R4 TH400 Swap? OldCarBum C3 Tech/Performance 14 05-27-2016 06:48 PM
    Rear End Ratio ? OldCarBum C3 Tech/Performance 34 04-15-2016 01:56 AM
    TREMEC - Let's beat a dead horse Black_Magic C1 & C2 Corvettes 13 01-06-2008 10:42 PM
    Need a little help with my tranny & rear-end experimentů benelliwang C5 Scan & Tune 2 06-12-2005 08:02 PM
    C6 Performance Ring and Pinion Mod 7 OUT C6 Tech/Performance 18 11-10-2004 01:18 AM


    Tags
    308, 390, 502, big, block, c5, corvette, end, ends, gear, gears, ratio, ratios, rear, speed

    Thread Tools Search this Thread
    Search this Thread:

    Click for Advanced Search

    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
    Trackbacks are Off
    Pingbacks are Off
    Refbacks are Off

    Forum Jump


    All times are GMT -4. The time now is 12:56 AM.