Find the Rearend Gear Ratio

Q: Find the Rearend Gear Ratio

Does anyone know how to find the rearend gear ratio on a 1967 firebird. (Besides pulling the cover off and counting teeth.) Any help is appreciated.

A: Take a piece of chalk and mark a line from the driveshaft yoke up to and on the differential housing. without moving the tire pick a reference spot on the car body near the rear tire (a screw or bolt, etc.) and mark the tire at this reference spot. Now, slowly rotate the tire while counting the number of times the drive shaft rotates past its’ reference mark. The rear gear ratio is the number of times the drive shaft rotates divided by the number of rotations the wheel rotates. example: drive shaft: 17, tire: 5… 17/5=3.4 so the rear gear ratio is 3.4:1. Good luck.

A: Look on the rear facing side of the left hand (driver’s side) half of the rear axle, about half way between the drum and the pumpkin. There should be a two letter code there. You may need a wire brush to clean it off to see it. Here is how you decode:

POSI CODES OPEN CODES
UN 2.56 XB
UP 2.78 XC
UR 2.93 XD
US 3.08 XE
UT 3.23 XF
UV 3.36 XG
UW 3.55 XH
UX 3.90
UY 4.33

Also, Here are the gear ratios for TH400 and Powerglide:

Power Glide 1st = 1.76 2nd = 1.00 Rev = 1.76
TH400 1st = 2.48 2nd = 1.48 3rd = 1.00 Rev = 2.00

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