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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: aaronboss221 on October 28, 2015, 01:32:44 pm

Title: Read end ratio question for 700r4 swap
Post by: aaronboss221 on October 28, 2015, 01:32:44 pm

Hello from Cookeville, TN. I have a 1986 c10 350 th350 combo with 2.73 gears currently running 255/70/15 tires. The motor has no upgrades done to it. I bought a 700r4 and was going to swap it out for mine. Just a little unclear on gear ratios and how it would effect my driving with the gears I have and tire size. I do have opportunity to buy a posi rear end with 3.73 gears off a local guy. Just wondering what I should do?(http://images.tapatalk-cdn.com/15/10/28/209f651a6ec3f7147b136355613ebd55.jpg)

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Title: Re: Read end ratio question for 700r4 swap
Post by: rich weyand on October 28, 2015, 03:12:13 pm

Don't put a 700R4 on 2.73:1 gears.  Your final in OD will be 2.0:1, which is ridiculous.  700R4s don't like to be lugged.

The 3.73:1 rear end with posi is perfect for the 700R4.  Do it.

Title: Re: Read end ratio question for 700r4 swap
Post by: aaronboss221 on October 28, 2015, 07:32:31 pm

That's what I was thinking but wasn't sure cause I had bigger tires on it. I appreciate it!

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Title: Re: Read end ratio question for 700r4 swap
Post by: rich weyand on October 28, 2015, 07:39:57 pm

Bigger tires lowers your rear end ratio further.  Think about it.  Even more important to get the 3.73.  Also, you'll like the posi.

Title: Re: Read end ratio question for 700r4 swap
Post by: Captain Swampy on October 29, 2015, 09:38:44 pm

Search for a gear ratio calculator, then you can play with tire size and gear ratio to get what you want. Bigger tires on the same gear = a taller (numerically lower) rear end ratio. The 3.73 would be the right direction, you could change tire size to work with those gears if it isn't exactly what you want.