'67 12 bolt gear ratio
 

I have a '67 Chevy SWB frame with a 12 bolt....trying to figure the ratio, best I can tell it comes out to be a 340. I don't know if it's really a 336,342 or 355. Guess I'll have to pull the cover off. I think it was an I6.

Re: '67 12 bolt gear ratio
 

If your count is correct it is probably a 3.42. I have not heard of a 3.55 or 3.36 used on GM trucks.

Re: '67 12 bolt gear ratio
 

My truck ['68 C/10 Stepside] came with a Factory 292 L6 and a 3.73 rear end ratio.

Re: '67 12 bolt gear ratio
 

thanx

Re: '67 12 bolt gear ratio
 

Factory i6, should be 3.73 which is an awesome ratio

Re: '67 12 bolt gear ratio
 

3.40 was a factory gear set in the mid 70’s for a couple of years. They are not super common, but you come across them. 34 teeth on the ring gear and 10 on the pinion. Not 3.42 ratio like you would find in a 10 bolt. As mentioned above, 3.73 would have been the standard gear in your truck. Any option would be listed on the SPID.

Re: '67 12 bolt gear ratio
 

I know in 50 plus yrs It could’ve been changed, but you could always check the code on the axle tube. Should be on the passenger side between the trailing arm mount and carrier housing. It usually anywhere from 12 to 3 o’clock on the tube.

Re: '67 12 bolt gear ratio
 

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If you find the code on the axle tube or housing here are the conversion charts I posted a few years ago.

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Check posts #8 & #19 for a couple of the code stamp locations..
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Re: '67 12 bolt gear ratio
 

If you don't want to pull the diff cover you can use these two calculators.

Get you tire size https://tiresize.com/calculator/
To find you gear ratio take the truck out and use your GPS to get an accurate MPH and a tach to get your RPM in top gear at like 70mph. Then put all that info onto
https://www.summitracing.com/popup/c...iAAEgIFzvD_BwE