I was wondering if anyone could help me determine my reag gear ratio.
I looked at the tech equations in the tech articles section but im not sure about my tire radius. The tires are stock size, with 16 inch rims. The car does 60 mph at approximately 2600-2700 rpm in 3rd gear (1:1 ratio) with the converter NOT locked up. (its a Hughes 3000, And i dont have the TCC lockup kit hooked up yet). I dont have the car with me so I cant put up the rear and do the driveshaft rotations thing. I just need an estimate, as I want to go with 3:73's or 4:10's and I want to make sure I dont already have one of those. The previous owner did install a posi rear but im not sure if he kept the 2:73's or what he used. Btw the car is originally an 87 formula 305/700r4

Thanks a lot,
Dave

 

if it's Carb I believe you will have 2.73... if FI, you will have 3.23 ... you can check the tech articles in the home section of this forum....


https://www.thirdgen.org/1987-pontiac-firebird

 

Without taking a peek, I doubt that you can be sure. The following from the 10-Bolt FAQ:

What Gear Ratio Do I Have?
Jerk the diff cover. The numbers are stamped on the ring and pinion. The RPO’s are usually found in the center console:
41 : 15 = 2.73 (RPO GU2)
41 : 14 = 2.93 (RPO GU3)
40 : 13 = 3.08 (RPO GU4)
42 : 13 = 3.23 (RPO GU5) / Approx. RPM’s at 70 mph--2133
41 : 12 = 3.42 (RPO GU6) / Approx. RPM’s at 70 mph--2255
41 : 11 = 3.73 (RPO GT4) / Approx. RPM’s at 70 mph--2463

Or from Randy’s Ring and Pinion:
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posi’s and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

Axle tube codes can be found in the following link (though rust usually makes them indecipherable, and they, like the RPO codes, indicate how the car was equipped new, and not necessarily what’s currently in place): http://sethirdgen.org/reardata.htm

JamesC