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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: Budget78 on February 13, 2010, 12:25:36 pm

Title: Gear Ratio?
Post by: Budget78 on February 13, 2010, 12:25:36 pm

Trying to figure out my rear end gear ratio, spun the rear tire, counted the revolutions on the drive shaft, came up with 3.11? Not sure that is correct, the axle serial number is KJU G3471. Would I be better served with a 700R4 compared to a 350. Could someone help me with this please.

Thanks
Jake

Title: Re: Gear Ratio?
Post by: oldredtruck on February 13, 2010, 01:05:01 pm

 what yr etc...1/2 ton - 3/4 ton?
did you check the glovebox door for codes etc...?

Title: Re: Gear Ratio?
Post by: VileZambonie on February 13, 2010, 02:20:22 pm

Remove the cover and check

Title: Re: Gear Ratio?
Post by: Josh454 on February 13, 2010, 03:34:05 pm

You were able to count the driveshaft revolutions down to .01? 

The only accurate sure fire way is to pull the rear cover.  Does the fluid need to be changed anyways?

Title: Re: Gear Ratio?
Post by: zieg85 on February 13, 2010, 04:10:54 pm

3 Standard rear ends in a 1/2 Ton around what you state are 2.73, 3.08, and 3.42.  I had a 3.08 in a 92 with a 700R4 or whatever GM used in a 1/2T for 92 and on a slight incline doing 55 it didn't know what gear it wanted to be in.  Just my $.02

Title: Re: Gear Ratio?
Post by: Irish_Alley on February 13, 2010, 10:55:40 pm

the only sure way to tell is pull the cover as stated before but if you have a tach you can use a formula or web site
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

http://www.idavette.net/tech/ratioc.htm
http://forum.73-87chevytrucks.com/smforum/index.php?topic=10879.0

Title: Re: Gear Ratio?
Post by: Budget78 on February 14, 2010, 01:11:11 pm

Thanks Guys

It is a '75 3/4 ton 4WD

Thought there would be a faster way then pulling the cover. It does have brand new fluid in it.
Does not have working tack. Does anybody know if axel serial number will reveal ratio? KJU G3471

Looking to get better gas mileage than what a turbo 350 can give me. RPM feels too high for the 55-65 MPH range.

Thanks
Jake  

Title: Re: Gear Ratio?
Post by: VileZambonie on February 14, 2010, 01:19:49 pm

http://www.73-87.com/chevy_ids/rpo-eg.htm#rpo-g

Look at the RPO in the glove box

Title: Re: Gear Ratio?
Post by: Irish_Alley on February 15, 2010, 12:41:24 am

the only thing about rpo codes is that it only true if the glove box is original and the rear is also. but it almost sounds like its a 3.08

Title: Re: Gear Ratio?
Post by: Blazin on February 15, 2010, 09:28:25 am

As far as i know the only ratios offered in that vintage 3/4 ton truck was 3:73's 4:10's, and 4:56's.
4;10's being the common one. Out of all the trucks I have owned, and / or parted out over the years, have only seen a handfull of 3/4 tons with 4:56's, and about double that amount with 3:73's.

Title: Re: Gear Ratio?
Post by: Captkaos on February 17, 2010, 04:10:08 pm

Ditto on what Blazin posted.

Title: Re: Gear Ratio?
Post by: Irish_Alley on February 19, 2010, 12:10:54 pm

right but i was going on what he said he calculated his was