73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Cowman85 on February 27, 2009, 05:02:41 pm

Title: 86 c20 rear end
Post by: Cowman85 on February 27, 2009, 05:02:41 pm

i was wondering how to find out what gearing i have in the rear end, i dont have any manuals yet unfortunately either.
its the th400 tranny, 350 4bbl, no power but a/c and cruise if that helps at all.
vin: 2GCFC24L6G 1190340
also i looked up the code on the engine and cant seem to determine if its the lt9 or hopefully the ls9.

Title: Re: 86 c20 rear end
Post by: fitz on February 27, 2009, 06:00:02 pm

Is the Factory code sticker still on the inside of the glove compartment?
The sticker will have an axle code that starts with the letter G.
GU6= 3.42 gears
GT4= 3.73 gears.
Those are the 2 of the common ones I know off the top of my head from scouting out parts trucks. If it is something else let us know and we can figure it out.

Title: Re: 86 c20 rear end
Post by: Captkaos on March 01, 2009, 07:02:08 pm

Pull the cover (probably need to chang the old oil anyway) rotate the ring gear and find the numbers on it  XX/XX and divide.

Title: Re: 86 c20 rear end
Post by: hilton850 on March 02, 2009, 02:57:58 am

what he said ^

Title: Re: 86 c20 rear end
Post by: Irish_Alley on March 02, 2009, 03:01:36 am

or if you have a tac

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
now saying that i have 4.53 my speedo is off by 3 mph

or you could use this web site
http://www.idavette.net/tech/ratioc.htm
found them after this whole ideal about the formula

Title: Re: 86 c20 rear end
Post by: Cowman85 on April 01, 2009, 08:22:20 pm

thanks for the info guys, i'll let you know this weekend when i finally pick her up from the shop!

Title: Re: 86 c20 RPO ident
Post by: Cowman85 on April 05, 2009, 01:31:01 am

turns out they are the GU6 - 3.42
i do have the LS9  :)
looking into the code MX1 to see if i have the TH400 (like the guy said) or the TH350.

Title: Re: 86 c20 rear end
Post by: eventhorizon66 on April 05, 2009, 07:34:09 pm

Check this out for transmission identification:

http://forum.73-87chevytrucks.com/smforum/index.php?topic=11124.0

Title: Re: 86 c20 rear end
Post by: Cowman85 on April 05, 2009, 11:30:29 pm

its a 400, is there that much of a noticeable difference between the 350 and 400? fuel wise and performance wise?
i read that the 400 robs more power from the engine, the 350 is lighter and one less plate so its also faster, anyone know about this stuff?
thanks

Title: Re: 86 c20 rear end
Post by: Cowman85 on April 05, 2009, 11:56:14 pm

does anyone know how much of a difference a transgo shift kit make with the th400?

Title: Re: 86 c20 rear end
Post by: Captkaos on April 06, 2009, 04:08:40 pm

If you are going to be towing the TH400 should be your choice.  It does use more power to turn, but it is also stronger.
I would add a shift kit to any trans personally.