Author Topic: Rear end gearing  (Read 2829 times)

Offline 87 K20

  • Newbie
  • Posts: 23
Rear end gearing
« on: October 28, 2011, 03:07:09 PM »
Hi all... I have an 87 K20 Longbed...I had 33s on it and just upgraded to 35s...it seems like I lost some power when I changed tire sizes, and doesn't that have to do with the rear end gearing?  Can anyone tell me how I can verify what gearing I currently have in the rear end, and also what the recommended ratio is for 35" tires?

thanks,

Mike

Online zieg85

  • Carl
  • Senior Member
  • ****
  • Posts: 7279
    • 73-87 GM squarebody extended cab and conversions up to 91 R/V series
Re: Rear end gearing
« Reply #1 on: October 28, 2011, 03:42:27 PM »
Look for the rear end code either on the glove box door.  You are looking for a 3 digit code, 2 letters and a number starting with either a G or H.  Post your findings and I can tell you what originally came in your truck.  The only true way though is to pull the differential cover and see what is stamped on the ring gear in case it was changed before you got it.
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Rear end gearing
« Reply #2 on: October 28, 2011, 08:16:07 PM »
with 35" tires, i hope your rear is atleast 3.73 or 4.11 or something similar.

http://www.4x4offroads.com/gear-ratio-chart.html


Offline bake74

  • Senior Member
  • ****
  • Posts: 5871
    • Build Thread
Re: Rear end gearing
« Reply #3 on: October 28, 2011, 11:20:03 PM »
     Also to note, don't change just the rear gears on a 4wd and not the fronts.    Otherwise  you will look like this after either your front or rear gears go boom.
« Last Edit: October 28, 2011, 11:24:21 PM by bake74 »
#1: The easiest and most obvious solution to any problem is 99% of the time correct.
#2: There is no such thing as impossible, it just takes longer.
  74 k10, 77k10    Tom

Offline Irish_Alley

  • Tim
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 13273
  • Family is not an important thing. It's everything.
Re: Rear end gearing
« Reply #4 on: October 29, 2011, 12:12:34 AM »
the formula for rear ratio is
if your running 50 mph @ 2k w/ 38" then
your ratio would be about 4.53
rpms x tire size / 336 / mph = ratio
2000 x   38    / 336 /  50    = 4.53

or you could use this web site
http://www.idavette.net/tech/ratioc.htm
found them after this whole ideal about the formula
If you canít tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline 1980c10

  • Senior Member
  • ****
  • Posts: 1205
Re: Rear end gearing
« Reply #5 on: October 29, 2011, 01:13:22 AM »
Not to sound too obvious; regardless of gearing going to a larger heavier and taller tire you will lose noticable power. Also if you feel that you don't have enough power than you'll need to change the gearing, then it becomes important what you have so you know what to get. IMO there is nothing like trying it out to see if it works. 

Offline 87 K20

  • Newbie
  • Posts: 23
Re: Rear end gearing
« Reply #6 on: October 29, 2011, 08:04:40 AM »
Thanks for the info I guys...I really appreciate it.  I forgot to mention that the truck is actually a half ton from the factory, but it has 3/4 ton running gear and suspension, so I'm going to have to pull the cover to see whats in there.  thanks again.

Mike

Offline westsidek20

  • Registered Users
  • *
  • Posts: 130
Re: Rear end gearing
« Reply #7 on: October 29, 2011, 01:55:42 PM »
I saw the chart and read the article, im a little slow  :P
So if he choose a 4.10 gear ratio for the 35" tires it will go faster then choosing a 4.88?

Online zieg85

  • Carl
  • Senior Member
  • ****
  • Posts: 7279
    • 73-87 GM squarebody extended cab and conversions up to 91 R/V series
Re: Rear end gearing
« Reply #8 on: October 29, 2011, 02:01:08 PM »
I saw the chart and read the article, im a little slow  :P
So if he choose a 4.10 gear ratio for the 35" tires it will go faster then choosing a 4.88?

Faster top end speed yes.  4.88, the engine rpms would be faster at the same mph speed if that makes sense
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline westsidek20

  • Registered Users
  • *
  • Posts: 130
Re: Rear end gearing
« Reply #9 on: October 29, 2011, 02:30:41 PM »
which one would get me moving faster from a stop?

Online zieg85

  • Carl
  • Senior Member
  • ****
  • Posts: 7279
    • 73-87 GM squarebody extended cab and conversions up to 91 R/V series
Re: Rear end gearing
« Reply #10 on: October 29, 2011, 02:37:31 PM »
4.88 but if you drive much highway your engine rpms will be high depending on tire size
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline westsidek20

  • Registered Users
  • *
  • Posts: 130
Re: Rear end gearing
« Reply #11 on: October 29, 2011, 03:10:23 PM »

Its just really confusing for me, but now i have a better idea.
I just finished my 383 and i was really surprised that i cant spin the tires (33x12.5X16.5) and a friend told me that i might need to change gears for that. oh my truck is a 1978 K20 with NP203 FF 14bolt rear end (havent figured out the gear ratio)

Online zieg85

  • Carl
  • Senior Member
  • ****
  • Posts: 7279
    • 73-87 GM squarebody extended cab and conversions up to 91 R/V series
Re: Rear end gearing
« Reply #12 on: October 29, 2011, 03:33:06 PM »
You more than likely have a camshaft that doesn't wake up until higher rpms are reached leaving your off the line acceleration disappointing, been there done that...  My experience was in an Oldsmobile Cutlass way back when I was 16.  The stock Rocket 350 2 bbl would roast the tires.  A friend of mine had a 68 Cutlass that had a fairly built Rocket 350 that his dad was making him sell because of his 3rd speeding ticket.  Anyway the cam sounded real BA and the car really moved out.  I did an all-night switch between the 2 engines, had them both running by 9 am.  My Cutlass was a dog that sure sounded mean but from a 30 mph roll ate anything I came against.  I ran it at Olds Day at Great Lakes Dragway in Union Grove Wisconsin.  I ran consistent 17.1 in the quarter mile at 105 mph.  I later did some research and found my Cutlass had the 2.56 gear stock and the 68 had a 4.11 posi.  Needless to say the 68 was really fun to drive off the line.  At the time I didn't know how that all worked and sold the 68 with the 4.11.  In hindsight it was probably a good thing I didn't put the 4.11 in mine as I would have probably gotten into trouble with it.
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Rear end gearing
« Reply #13 on: October 29, 2011, 03:34:51 PM »
Quote
Its just really confusing for me, but now i have a better idea.

think about NASCAR; at Talladega (2.66 mile track), the cars use something like a 2.50 :1 rear ratio.  They will near 200mph, but, they take two laps to get there because the ratio is so high (omitting the restrictor plate factor )
If, at Talladega they use a 6.00 :1 ratio like they use at Bristol or Martinsville (1/2 mile track), then the cars could reach top speed in about 1/2 lap at 'dega, but, their top speed will be so much slower becasue rpm's will be screaming. 

If they use a 2.50 : 1 rear at Bristol, their cars would be such a dog and never able to accelerate completely before going into another turn where they would decelerate to make the turn. All other cars would pass them in half a lap because they couldn't take advantage of their 9,000 rpm engines.  Their rpms would never reach 3,000 with a high rear ratio.
« Last Edit: October 29, 2011, 03:36:48 PM by SgtDel »