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Knowing your Gears

by Ziyad Showket AKA Zipp

How to calculate your gears
Gear calculations are used for determining the RPM or the speed you want to attain under a specific circumstance.  If you are running a quarter mile you want to be at your peak power in your top gear at the end of the track.  That way you know you have gotten the most out of your acceleration down the strip.  We have all heard people throw around numbers like 4.11s or 3.73s, but what do they mean?  4.11 means that for every 4.11 times the drive shaft turns, the tires turn once.

Tall gears VS. Short gears
Taller gears (numerically lower numbers) will make the engine turn less RPM at any give speed.  With taller gears the engine also has less mechanical advantage over the tires but will increase fuel mileage.  Shorter gears (numerically higher numbers) will make the engine turn a higher RPM and any given speed.  The engine will have more mechanical advantage over the tires.  The more mechanical advantage the engine has over the tires, the faster the vehicle will accelerate, but the engine will turn a higher RPM at sustained speeds reducing fuel mileage.

How to pick your gears
Your gear selection should be determined by how you are using your truck.  For example, if you tow a lot with high weight in an area with a lot of hills, you should look for Short gears (4.56).  If you do not tow and live in a flat are then you do not need short gears, you can go with taller gears for better mileage (3.08).  But, if you are drag racing all the time and want to be able to light up your tires then shorter gears are for you.

Lets say you lift your truck and replace your stock tires with 35s.  Most stock tires measure about 31.5” tall.  So there is a difference of 3.5” in total tire height.  This is roughly a 12% change in tire height.  If your truck came with a gear ratio of 3.73 you now have effectively changed it to 3.28.  (we are assuming 1 to 1 final drive for argument).  You will notice your truck is a little more sluggish out of the hole and on the freeway your RPM is a little lower (yup, by 12%).  The way to compensate for this change is to replace you 3.73 gears with 4.18.  Now, the odds of there being that specific ratio are slim to none but there are 4.11s, close enough to be back to where you were.

What if you want more out of your gears than just stock?  Well, you will need to calculate what you want your truck to do and when.  Using 1 to 1 as a basis here is the formula for calculating the change in gearing:

In other words, if you want to know what your RPM will be at 60mph with a gear ratio of 3.73 and 35” tires your formula will look like this:

  Now, lets throw in another variable, Over Drive.  Almost all new cars and trucks have overdrive in the transmissions, whether auto or manual transmission.  The most common range of over drive is anywhere from 28 to 32% (0.72 to 0.68).  In order to get a proper analysis of your new gear configuration you will need to multiply your final answer from the above formula by your over drive amount. 

You will notice that the change in gearing is not affected as much in overdrive as it is in a 1 to 1 scenario.  Lets say for the sake of argument you change your gears from 3.73 to 4.11, this is about a 10% change.  But once you factor in your over drive – say 30% (.70) your change is now 10% change x .70 overdrive  = 7% change.

Below is an Excel spreadsheet you can download and play with to help you to select the proper gears for your truck.

Zipp

Spreadsheet below was authored by Zipp - updated and more parameters added by Kelly Portz 

<<< DOWNLOAD SPREADSHEET HERE >>>