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Gear ratio identification?

3K views 16 replies 4 participants last post by  Chevymann74 
#1 ·
I'm getting ready to buy this early 70s nova 12 bolt diff and according to the guy its already posi and has some work done to it seems like it might have its got a nice aluminum cover and both wheels spin the same direction I checked the numbers on the diff itself and it says its an open diff with 3.31 is there any way to find out exactly what it is does it have casting numbers?
 
#4 · (Edited)
The ratio should be stamped on the ring gear itself, well the number of pinion and ring teeth are, you'll have to calculate the ratio by dividing the amount of pinion teeth into the amount of ring gear teeth. If it is now a posi and was not originally the number stamped on the passenger side tube will be irrelevant anyway. You can also turn the pinion and count how many revolutions it takes for one full turn of the axle.
http://www.chevellestuff.net/qd/generic_rear_axle_info.htm
 
#6 ·
We need to no a bit about the engine and tranny as well as your driving, whether it's highway or around town, or a combo. I ran 331's behind a 435 hp BB and 4 spd it would have bee a great gear with an automatic. I went o a 373 gear and love it, although it's not overly highway friendly, but still reasonable. A 355 is a great choice for a 3 spd auto or 411 with a 4 spd OD tranny.
 
#12 ·
I think you'll enjoy the 355 more, most of my driving is around town and a bit of highway. I think if I was driving 50/50 city, highway, I'd rather have the 355.
 
#14 ·
Be careful

Choosing a gear ratio is picking your fuel economy too. My 383 sbc, th350 has a 3:73 posi and 29" tires. My rpm at 60 is over 3000 which is considered high nowadays. The car does do better mpg than my boat that has twin 4.3 v6's, and gets about 2.5 mpg at 27 mph, and the car is not my daily driver, so fuel economy isn't a goal for me.
If most of what you are going to do is drive it around town and quick acceleration isn't a big deal, go for the higher gears. If you want to be the fastest guy at an intersection go for some really low, like 5:3's, but your top speed may be 40 mph. It's all about how you are going to use the car.:cool:
 
#15 ·
Choosing a gear ratio is picking your fuel economy too. My 383 sbc, th350 has a 3:73 posi and 29" tires. My rpm at 60 is over 3000 which is considered high nowadays. The car does do better mpg than my boat that has twin 4.3 v6's, and gets about 2.5 mpg at 27 mph, and the car is not my daily driver, so fuel economy isn't a goal for me.
If most of what you are going to do is drive it around town and quick acceleration isn't a big deal, go for the higher gears. If you want to be the fastest guy at an intersection go for some really low, like 5:3's, but your top speed may be 40 mph. It's all about how you are going to use the car.:cool:
Well im just looking for around street some highway driving and when I wanna get up and go I wanna go with good top end and low end power lol what is best for me?
 
#16 ·
I stepped out for a bit!:D

As mentioned, 373's will reduce the fuel milage and take a bit away from highway driving, but I figure I only use my car for fun, so I wanted the lower 373 gears over 355. I was going to go with 411's, but I'm glad I chose the 373's now.

If milage isn't a concern and your driving 80/20 more city, the 373's are more fun for spirited driving.
 
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