no idea on gear ratio s , how u figure them

[73charger440]

ive allways heard others talk about the differnt rearend and   gear ratio s but have no idea on what the hell they are talking about ,im needing to buy a rearend for my car i have a 8 3/4 rearend and am wanting to just replace the hog head part of the rear end but have no idea what to buy .im on a very tight buget i m wanting a possi rearend  have no idea on gear ratio ,ill be using the car for shows maybe a couple times down the strip ,cruises are a must the miotor im hoping for about 450hp  out of if .thanks fro any input on a ratio and rear end type

[Ghoste]

Get a 3:55 or maybe even higher if you can get your hands on a 3:23.  Your 8.75 will work fine and there are plenty of limited slip options for it.
The ratio can be thought of as a big lever for your car and the different ratios are like moving the pivot point.  In some cases you will lift a weight easier but less distant and others will move it further but more effort is required.  What this means is that a high ratio like 3:23 will allow the car to move down the highway at a lower rpm but it won't accelerate into the powerband as quickly at the dragstrip.  A lower one like a 3:55 does the opposite.  Also, a lower ratio will mean poorer gas mileage as the engine will be operating at higher rpm more of the time.

[John_Kunkel]

It would be a good idea to determine what your current gear ratio is before making plans to change it.

To determine your current ratio, jack one tire clear of the ground and, with the transmission in Neutral, turn the tire TWO complete turns while counting the number of driveshaft turns. The number of driveshaft turns is the gear ratio. IOW, if the shaft turns about 3 1/4 turns the ratio is 3.23...if it turns about 2 3/4 times the ratio is 2.76.

[73charger440]

one more question is a limited slip the same as a possi rear end  ,and are all possi rear ends dana rear end s ,ive heard if u jack up ur car and turn one wheel and the other side goes in the opposie direction its a nonpossi rear end ,and is the ratio stamped on the gear its self.u know that was more like four more quesitons lol .thanks

[Ghoste]

Posi is a nickname for Chevrolet's Positraction and yes it's a limited slip rearend.  Chrysler called their limited slip a Sure Grip.  Each of the automakers has had a name for it for marketing purposes.
Assuming the Sure Grip hasn't worn out completely the yes, both wheels whould turn in the same direction.
Yes, the ratio should be stamped on the ring gear someplace.
The Dana 60 is a rearend that was supplied to Chrysler by Dana Corp and not unlike "posi", "Dana" has become a nickname for heavy duty Chrysler rearends right or wrong.  The Dana 60 that Chrysler supplied for their musclecars were all limited slip.
If you are looking at a used Dana, keep in mind that Dana supplied several different axles to a number of different manufacturers.  The Dana 60 with 9.75 inch ring gear is the one that you want for a high output musclecar (and even at that, there are different widths for different applications).  If you are looking at aftermarket Dana's they are nearly all the desired 9.75 one.

[flyinlow]

The ratio was  stamped on a small metal tag under one of the bolts, if is still there after 40 years.


                                        Gear Ratio to 1                             RPM @ 60MPH                       Originally in / gas milage

High gear ratio (tall geared)          2.76                              2000-2500                             2 bbl standard cars,station wagons, New yorkers,etc.
                                                                                                                              best gas milage

Base performance/towing              3.23                              2400-2800                            Base axle in Road Runners, Charger RT's, etc.
                                                                                                                            option for improved towing in most cars

Performance Optional axles          3.55,3.73,3.91                   2800-3400                           Performace car options for increased acceleration
                                                                                                                              gas milage suffers

low gear ratios                      4.10,4.56                        3200-3800                                special order/aftermarket for mostly track cars.
                                                                                                                               poor gas milage              

Race axle                                 5.13                       Not driven at freeway speeds            race cars and dump trucks
                                                                         without overdrive for long



RPM's vary with tire size and are approximations for illustration purposes.


I have a 3.55 in my Charger.

[jeryst]

It would be a good idea to determine what your current gear ratio is before making plans to change it.

To determine your current ratio, jack one tire clear of the ground and, with the transmission in Neutral, turn the tire TWO complete turns while counting the number of driveshaft turns. The number of driveshaft turns is the gear ratio. IOW, if the shaft turns about 3 1/4 turns the ratio is 3.23...if it turns about 2 3/4 times the ratio is 2.76.

I always thought it was 1 turn of the wheel. Why 2?

[Memphis Mopar]

 

        I thought it was only 1 turn of the wheel also

[flyinlow]

One turn if you have a suregrip and both wheels can turn. If you have an open diff. leave one wheel on the ground and use two turns of the jacked up side. If you try it with an open diff. and both wheels are off the ground ,you have to make sure they both spin together to use the one turn method. Some times the brakes dont drag the same all the way around and you can get an inaccurate number with both wheels free. One wheel will go part way around and stop or slow and you dont notice and you gear ratio will be off.