Hi Gary and welcome to the site. Since it sounds like things have been changed on your truck, you may have to remove the rear cover of the differential and either count the teeth of the ring and pinion gears and divide them to see what you have or maybe you could see the stamped code on top of the axle tube on the passenger side of the differential assembly. But...if the ring and pinion were for some reason changed in your transmission, that would alter things. As memory serves I think 3.90 was the ratio most commonly used on the 1961 K10. You can also only jack one wheel up, mark the driveshaft and rear end with a bit of chalk so you can count the turns and rotate (transmission in neutral) how many turns it takes to equal 2 rotations of the free wheel. However many turns it takes to rotate the wheel one time will tell the story. Almost 4? Could be 3.90 or 4.11, depending on how accurately you did this. It isn't exact...but it will get you into the ball park. Note: if you have a worn-out positraction rear end this method may not work. Depends on how worn out it is.
