Gear ratio?

[80 Chevy K20 4x4]

I'm trying to figure out the gear ratio in my brother's 79 4x4 without pulling the diff cover. I jacked up the rearend and marked the rear driveline and the rear tire. How many times do I need to rotate the tire to get the ratio figured out?

Thanks,
Doug

 

[big83chevy4x4]

rotate the tire 2 times (with only 1 tire jacked up) and count the turns on the drive shaft.

 

[80 Chevy K20 4x4]

I marked the inside of the tire (33") and rotated the tire two revolutions an counted the turns off the driveline(marked as well) and it was over 3.25 turns but under 3 1/2 turns. So I'm assuming that it's most likely 3.42 gears, or would it be 3.08's? I drove it down the county gravel road at 60 mph according to the speedo, and it ran 2200 rpm. That's assuming the speedo is accurate. Any ideas?

Thanks,
Doug

 

[4X4HIGH]

More than 3.25 turns but less then 3.5 turns would make them 3.42's since the only other gear would be either 3.08 or 3.73 of which both are either less than or more than your 3.25-3.5 figure.

 

[tRustyK5]

The math confirms 3.42's as well. Should be running at 2207 rpm at 60 mph...


Rene