Re: [Chevelle-List] 12 Bolt Rear End

[Emil Dusek]

Keith, tire size only comes into play when calculating your EFFECTIVE gear ratio. Most street type tires are ~26" tall so if you change to a taller/shorter tire then you have effectively changed your rear gear ratio. Your tire/wheels are bolted(wheel studs)to your axles so they absolutely rotate together. Emil Dusek:71Chevelle SS 502     Keith Cooper wrote:   If you use the drive shaft rotation method, should you look at the axle rotation and not the tire rotation?? If the tire size ( 14" to 15" or 16" rims and tires) has been changed then the tire rotation ratio would be wrong I think, wouldn't it?? I would think that you need to look at the drive shaft to axles rotation. Am I wrong or right. This may be why my 71 Chevelle comes out to be 2.5 ratio which seem low. I have put 15" rims and tires on the car.Keith ----- Original Message ----- From: LXVIISS To: [EMAIL PROTECTED] Sent: Monday, February 04, 2002 10:05 PM Subject: Re: [Chevelle-List] 12 Bolt Rear End  Hello T in SC, You are basically correct about how to determine the rear end ratio. For a 3.73:1 ratio it's actually 3 and almost 3/4 (.75) revolutions of the drive shaft to one revolution of the tires. It equates to the number of teeth on the ring gear (driving the tires) divided by the number of teeth on the pinion (driven by the drive shaft). In gear design, it's really based on the pitch diameters of the gear and the pinion, but the number of teeth is a hell of lot easier to determine. For a 3.73 gear set, I believe there's 11 teeth on the pinion and 41 teeth on the ring gear (41 / 11 = 3.72727 rounded to 3.73). You can pull the rear end cover to count the number of teeth. But counting the revs for the drive shaft and tire should get you close enough. I don't know the standard ratios offered for 1969, but in 1967 they were 2.73, 3.07, 3.08, 3.31, 3.36, 3.55, 3.70, 3.73, 4.10, 4.56 and 4.88. I think some were just for 10-bolt rears, some were just for 12-bolt rears and some were just for automatic transmissions. My 1967 is a 12-bolt with 3.73 gears in it, but a prior owner had 3.07 gears in it for a while. I hope this helps. Mike ----- Original Message ----- From: Lojac To: [EMAIL PROTECTED] Sent: Sunday, February 03, 2002 10:56 PM Subject: [Chevelle-List] 12 Bolt Rear End  Can anyone tell me if there was a stock gear ratio for the 69 Chevelle SS396 rear end?  If not, can anyone tell me how to determine my rear end gear ratio.....aside from taking off the rear end cover or tearing the rear end down.  I am not quite ready to do that.  I heard that you could jack that car up and watch the rear u joint.  The number of times it spins while completing one full revolution of the tire can determine the gear ratio.  For example if the u joint turns 3 and a quarter turns then the rear end is most likely a 3.73.  Any ideas??? Thanks in advance,T in SC