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Discussion Starter #1
Hello everyone! I have a 68 nova. Running a 327 - 325hp and a Muncie M20 4 spd and stock tires. Wondering what the rear end gears could possibly be. On the highway doing between 60 - 65 mph she’s screaming at 3200 rpm! Any ideas on what ratio I have and what gears are good for long distance highway cruising??? Thanks
 

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https://www.blocklayer.com/rpm-geareng.aspx

here is a link you can play around with to figure out what gears you would like to install. you can figure out your rear gears your self. lift the back so both tires are just off the ground. turn the passenger tire one revolution forward an count how many times the drive shaft turns. trany has to be in neutral an front wheels blocked. so 3 an a 1/2 turns of the drive shaft would be a 3.55 rear.
 

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That would be two revolutions of the rear tire. Rear tire diameter plays a big role in what gear ratio you choose.
 

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Have you checked the rear cover to make sure it doesn't have a tag attached to one of the bolts. A lot of rear ends came with tags with information on them. Of course if the gears aren't factory it wouldn't matter. You can do the one tire revolution and count the driveshaft but you can also take the rear cover off and count the teeth on the ring gear and pinion and do a little simple arithmetic also. It would be a great chance to check/change the gear oil and check for wear/broken parts.
 

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Discussion Starter #5
It has a moser 12 bolt posi. So it’s not stock but I will jack it up and count the revolutions that way. I do like the idea of opening it up and checking things out and changing the oil though. I just thought that 3000+ rpm was high at 65 mph!!! It really makes for a stressful highway cruise haha.
 

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My inline 6 has, in the past, handled 3,000 - 3200 rpm for a long trip.

I resolved that issue by installing a 200-4R overdrive transmission.

Search for a formula on the internet, where you can plug in data, ie., tire size, transmission gears, etc, to see what rear gear set you currently have.
 

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Assuming your stock tires are around 26" tall, you're probably running 4.10's in there.


When I had 4.11's in my 9", in 4th (1:1) with my 26" tall tires it was about the same, right around 3K. 70 was close to 4K.
 

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I run 3000rpm at 60mph. I have 3.73 gears and 27 inch tall tires. I also run a 4 speed so no convertor slippage.
 

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The only other gears you can get for a 12 bolt to do what you want are 3.31 or 3.08.
 

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That would be two revolutions of the rear tire.
You are wrong, exactly one turn is correct as RedRider66SS explained. I've been doing it that way since the '60s, if you know the gear ratios used by the manufacturer you will know the ratio in your diff.

Mike
 

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We will have to agree to disagree. 1 turn for a posi or 2 turns for an open rear end. Done a few myself.
 

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Discussion Starter #12
I love how a relatively basic question for all the gear heads causes a huge conversation. I can’t imagine what would happen if we all got together and had some beers?? but I appreciate all the input. Keep the conversation going folks!!!
 

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When spinning tires for gear ratio you spin one time with a posi and both wheels off the ground. With an open rear end you spin the tire twice with one wheel off the ground and the other on the ground. Count the yoke turns.
 

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I like to use this formula: mph x final trans ratio x gear ratio x 336 divided by tire diameter = rpm.

So for yours 65 x 1 x 3.73 x 336 / 26 = 3,133 rpm.

If you go to a 28” tall tire rpm would drop to 2900 at 65 mph. Something to think about, could be the cheaper fix.
 
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