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Tuesday, January 22, 2013

Little Something About Gears and Speed

** Updated 18Feb2013: New reference towards the end.

Intro 

Just wanted to point out a math calculation related to ring and pinion gears.


Few items to keep in mind

For one, every vehicle and situation is different.  There isn't a one size fits all unless you are a auto manufacturer and are planning to make money.

Second, there is math involved.

Finally, its not a bolt on application.  Get help if you never installed gears in a differential before unless you don't mind spending a lot of time figuring it out and/or costing money in potential damage to your vehicle if the install isn't done properly.

So back to the math...the main reason I wanted to post this.  Two scenarios:

(1) a vehicle with 3.73 gear ratio doing 2000 rpms

(2) a vehicle with 4.11* gears ratio doing 2000 rpms
* lower gear (higher numeric value) than the 3.73 gear

To get the drive wheels/ring gear rotation per minute.

RPMs / Gear Ratio = Rotation per minute

So from the two scenarios above:

(1) 2000 / 3.73 = 536.2

(2) 2000 / 4.11 = 486.6

How about another:

(1) 3500 / 3.73 = 938.3

(2) 3500 / 4.11 = 851.6

Extreme gear:

(1) 4000 / 8.00 = 500

(2) 4000 / 3.27 = 1223.2

As you can see from the calculations, the gear used affects the rotation of the drive wheels moving the vehicle.  So, although you will gain more torque installing lower gears (high numeric values) your speed will decrease when compared with higher gears.  Assuming you have not change anything with the engine, your overall top speed will decrease.  This varies from vehicle to vehicle as not all vehicles have the same red line.  HINT: don't ride the line due to engine wear and potential damage.

Another example is the reverse gear vs forward gears.  The reverse is a lower gear ratio allowing faster acceleration, more torque (only have one reverse gear) but lower top speed before you get to the red line of the engine.

When determining a gear upgrade keep in mind that the more gear, the less top speed, but the more torque you will gain giving an increase in acceleration to the top speed of the vehicle, all limited by the ability of the engine.

Reference:
Car Gears Explained End-to-End Video
Car Craft: All About Axle Gears


Disclaimer: I'm not a gear pro.  I only know what I know.  So be sure to do your own research before making a decision.  I just wanted to share what I learned and put out there simple calculations to show the difference with changing gears.


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