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BWAL Is My Hero
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Choosing the Proper Gear Ratio

In this setup the owner has chosen to use a taller tire and 4.10 gears to take full advantage of his centrifugal supercharger.


Ok, here we go. This quick tip is not about which gear ratio is the best or what brand is the best. This is simply to point out the factors that contribute to the selection of the proper ratio for you, your driving style and your vehicles combination of parts. I know everyone has seen the titles of the threads “Should I get 4.10’s?” or “Which gear’s?”, among others. The following is a guide to helping you choose a ratio, not to tell you which ratio is best.

What factors affect the choosing of the proper ratio?

*Image Courtesy of Erin May*
Here you see a ring and pinion. Commonly referred to as “Gears”



What do I do with my car? There are many different factors that go into choosing the proper ratio. One of these is asking yourself; “Where does my car spend 90% of its life?”. Are you an avid drag racer? Do you spend your weekends on a wide open road course or cruising down the highway? How about driving around town idling in traffic with short stints from stop light to stop light? Each of these very different scenarios are very important when considering installing a new set of gears. A good example would be drag racers. For the most part a drag racer is going to want to stick with a gear that is short enough to keep him in the “peak power” RPM range but not so short that he is going to run out of RPM by the finish. Meaning, he wants to be able to go across the finish line without hitting the “Redline” or “Rev-Limit” but does not need much more. Conversely, someone who does a lot of highway driving is going to want a “taller” gear which will keep you in the lower RPM range and help your gas mileage on those long trips.

Tire Size

* Karen B (Aka NASTY03GT) *
Here you see the height difference between a drag radial (Left) and a street radial (Right). In this setup they have chosen a taller tire to likely compliment shorter gearing.


Tire size is a very important factor when choosing the proper gear ratio. By changing the outer diameter of your tire you can drastically change the “effective” gear ratio of your combination. Meaning, the farther you deviate from the factory tire size the larger the swing in “effective” gear ratio is going to be. If you have a tire that is physically taller than your factory tires your affective gear ratio changes upward. When you run a much shorter tire than the factory tires were you lower your effective ratio. For example: My car has the factory optioned 3.55:1 rear gear ratio. The factory tires were 235/55/17. My new tires are 275/35/17’s. The diameter of my new tires is 24.578740157480315 inches. The diameter of the factory tires is 27.17716535433071 inches. Which makes my “effective” gear ratio 3.93:1. To calculate this for yourself use this calculator to calculate your tire diameter: Tire Diameter Calculator and this one to calculate the change in gear ratio: Gears Needed After Adding Larger Tires . Now you can make a more informed decision before ordering that next set of meats!

Drivetrain Setup


Here you see a dragster with a powerglyde transmission. This would be an example of a car that is going to use intermediate gearing and a very tall tire to make up for only two forward gears.

Drivetrain setup is also very important. Things like: type of forced induction, type of transmission or when making changes to heads and cams. These all contribute to properly selecting gears. With a turbo combination it is generally recommended to stay with a taller gear which keeps you gears longer to take advantage of the mid-range and top end power that a turbo combination creates. Conversely, generally a centrifugal supercharged car is more likely to show better results with a lower gear so you can climb through the power band faster and get to peak where the centrifugal supercharger makes its best power. With a naturally aspirated combination, too, you would likely want to choose a lower gear to get you into the “power band” faster to take advantage of those neat new “bump sticks”, aka Camshafts, you just installed. Likewise, if you have replaced your factory transmission with a aftermarket piece you are going to want to consider that as well. Are you a drag racer that switched to a TH400 with no overdrive? May want to stick with a taller gear. Did you swap your stock Tremec 3650 for a Tremec 6060 GT500 style 6 speed? You may want to go with a altered gearing to take advantage of the altered gearing and extra gear in your new 6 speed.

Personal Preference

Here you see a plethora of Mustangs. Each with a different setup, however slight, suited to the tastes of its owner.


This is probably the most important factor, at least if you ask me. Some guys/gals just prefer a certain gear ratio because that is what they have used and that has what has worked for them. This kind of goes along with driving style. Some people like to row through the gears in their manual transmission as fast as possible, while others like to hang out in one gear excellerating for a long period of time. Even if it is not “optimal” pick a gear you think you are going to be happy with. Just like anything else, don’t let anyone talk you into something without first knowing the facts.

Written by Jacob McCune

Special thanks to:

Robrobinette.com for the great tire diameter calculator!

TheRangerStation.com for the gear ratio change calculator!

Also, Erin May and Karen B (Aka NASTY03GT) for the great images!
 
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