pontiac rear end

i have a 49-50 Pontiac silver streak and im wondering if any one knows that gears are in the rear? the rear is color red

 

Only way to know for sure is to pull the cover and divide the # of ring gear teeth by the number of pinion teeth

 

ahh ok, i saw the service book and it said some thing like 40:1 some high number like that

 

Just for a quick reference, lift the rear tires off the ground, mark 1 tire at either 12:00 or 6:00. Rotate the driveshaft 1 revolution and count how many times the marked tire rotates. You can get a fair idea of the ratio. Popular ratios would be 3:55, 3:70, 3:90, 4:11 or 4:56. In other words, when you turn the drive shaft 1 revolution and the marked tire turns 3 1/2 times, it's probably a 3:55.......

 

Shop manual says the red code is a 3.90, code ''3'' will be stamped in a raised pad front of carrier ... to verify, lift BOTH rear wheels off the ground, set the axle stands, mark both tires, turn each exactly 1 turn while counting the driveshaft revolutions.

 

so what ratio is 3.90?