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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: reeperc10 on December 20, 2015, 11:26:07 pm

Title: I need help on what gear ratio I have
Post by: reeperc10 on December 20, 2015, 11:26:07 pm

I have a 79 c10 stock motor was a I6 but I ended up putting a 350 in it. Can anyone help me what stock gears came in those trucks. I heard they some come with 342 or 373?

Title: Re: I need help on what gear ratio I have
Post by: zieg85 on December 20, 2015, 11:44:21 pm

My 79 GMC with a 250 I-6 had a 2.76 gear

Title: Re: I need help on what gear ratio I have
Post by: reeperc10 on December 20, 2015, 11:53:11 pm

That's what I thought I had. But I have different people telling different things. How did you find out what ratio you had

Title: Re: I need help on what gear ratio I have
Post by: zieg85 on December 20, 2015, 11:59:37 pm

I had the code sticker under the hood that told.  The only other way to know for sure is to pull the cover and see what is stamped on the ring gear.  Ratio's used 2.56, 2.76, 3.07, 3.42, 3.73 and 4.11 in 12 bolt rear ends.

Title: Re: I need help on what gear ratio I have
Post by: reeperc10 on December 21, 2015, 12:09:30 am

Thanks zieg85

Title: Re: I need help on what gear ratio I have
Post by: rich weyand on December 21, 2015, 01:06:51 am

If you have a tach, you can also tell from the rpm at a given speed.  Tell me your rpm at 60 (in high gear, but not overdrive) and I'll tell you your gear ratio.

Title: Re: I need help on what gear ratio I have
Post by: travisr1988 on December 21, 2015, 08:08:42 am

My '77 had a 250i6 and 3 on the tree, I used a gm options guide I found somewhere on the web and it listed 250/3 speed standard has 2 options, 3.42 and 3.73, although I'm not entirely sure that's accurate. If you don't want to pull the cover you can lift the rear end up and mark the driveshaft and 1 tire and count how many times the shaft turns for each rotation of the tire. Mine turned out to be 3.42.

Title: Re: I need help on what gear ratio I have
Post by: Irish_Alley on December 25, 2015, 03:10:55 pm

How to determine what ratio you have without removing the cover (http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.0)

Title: Re: I need help on what gear ratio I have
Post by: Blazin on December 26, 2015, 08:33:17 pm

Pull the cover, divide the last two numbers. Best way to sure what they are.