I have a 1964 Chevy Impala 327, I need to know the rear in gear ratio as I am going to put a Cam with  Edelbrock aluminum Performer RPM heads. Before I buy the cam I need to know what gear ratio I have.( I already have the heads..new). I was told to jack up the rear end tires off the ground and mark the drive shaft.Count how many complete turns the drive shaft turns when I rotate the tire one complete turn. I come up with the drive shaft turns 1 3/4 turns to one complete tire rotation.Does this sound right? Does that mean I have a 1:34 ratio.I don't think so. Should it be like 3:36 or 3:70? Any input would realy be appreciated.Also any ideas on cam?(don't want a shaker or change torque coverter but a noticable idle. Thank you  

Sounds like the differential is making the reading come out wrong. You would need to make sure both tires turn the same. On the ground works, but it is harder to watch the drive shaft. Another option is pull the cover and count the teeth in both gears and devide.  

The differential is probably making it come out wrong. You need both tires to turn the same. On the ground works but it is harder to watch the drive shaft. Another option is to pull the cover and count the teeth on both gears and devide.  

Well sorry im late, but does your Impala have limited Slip? Might sound like a dumb question to some Guru's around here, but to my amazement my buddy Nathen's 71' Chevy C-10 has limited slip. I could of swore it was a true posi. Well if so then the limited slip somehow throws the count off. My 91' Silverado 305 has limited slip and I could never tell the gears so I had to grab a few numbers off the Diff. and the Shafts and a few numbers from the vin and pulled it up online. My truck has like 2.73" gears in it I think. You might have like 3.73"'s or something in the area.  

May I ask what you think the difference is between a "positraction" and a 'limited slip' differential?  

the posi spins both tires at the same time, when the limited slip lets 1 tire spin while the outher side catches and moves until the outher side grabs traction.