Video: How to Select the Proper Gear Set for Your Vehicle

In this edition of Summit Racing Quick Flicks, Mike will teach you how to select the best gear set for your vehicle. Learn how to figure out what gear ratio you have and why it’s important, and how to select the best ring and pinion for your particular application.

Also, you can read our earlier post on how to choose a ring and pinion in four steps.

Hi I’m Mike and on the installment of Summit Racing Quick Flicks we are going to talk about selecting the proper gear set for your vehicle.

The selection of the proper gear set is critical the vehicles overall performance. It is important that the proper gear set is selected due to the fact that it’s going to have a drastic effect on the vehicle’s acceleration and cruise rpm. In this video, we are going to discuss how a gear set operates, how to determine what gear ratio is currently installed in the vehicle as well as the aspects on the vehicle that can effect on the gear set being selected and how to select the proper gear set for the rear-end that’s in your vehicle.

The first thing we need to discuss is how the ring and pinion operate. The ring and pinion is going to consist of two gears–your pinion gear, which is your drive gear and the ring gear, which is the driven gear. The pinion gear is what receives the motion from the driveshaft that is being transferred throughout the drivetrain. As that driveshaft spins, it spins the pinion gear at the same time. This pinion gear is then going to be connected to the ring gear and as the pinion gear rotates, it’s going to rotate the ring gear. This is then going to go ahead and put the axles into motion, then the wheels into motion to move the vehicle forward or backwards, depending on what gear the transmission is in. The relationship of these two gears is what has the effect on how the vehicle accelerates,  because the pinion gear is going to have X amount of teeth in relationship to the ring gear. This is what’s going to affect the vehicle’s acceleration and cruise rpm as an end result.

Often a point of confusion, there is only one way to accurately determine what ratio is currently installed in the rear-end of your vehicle. It’s a misconception that sometimes you can go ahead and count the number of tire rotations in relationship the driveshaft to determine what ratio is in the vehicle or that there is some sort of tag on the rear end that tells you what ratio is currently installed. The only way to determine what ratio is currently in the vehicle is to count the number of teeth in the ring gear and divide it by the number of teeth on the pinion gear. As an end result, this will give you the ratio that is in that vehicle. For example, if we had a ring gear that had 41 teeth on it and a pinion gear that had 11 teeth and divided the two that would give us a 3.73 gear ratio. The reason why it is important to understand what ratio is currently installed in the rear-end of your vehicle is because you need to determine which direction you need to go for your vehicle application in comparison to how the vehicle is operating now.

There are five basic aspects to the vehicle we are going to look at to determine which ratio is right for our specific application. The things that are going to have an effect on the gear selection are the tire size, because the tire height or the tire diameter can have a drastic effect on how the gear set reacts in the vehicle. The cam profile– every cam has an operating range, and we want to make sure the vehicle is going to accelerate properly to get into that cams operating range quickly enough or effectively. We are going to look at torque converter selection–what stall rating is the converter that is installed in that vehicle. Because what can happen is we can have a torque converter that say has a 3,500 rpm stall, but we have, let’s say, a 3.08 gear set installed in that vehicle with a fairly tall tire. What that does for us then is the vehicle comes to the cruise rpm, and  it never gets to the point where the converter actually gets into the stall of converter and the converter continually slips, overheats, and causes poor mileage issues and things of that nature. We are going to look at cruise rpm, which again, relates directly with that stall converter selection. We want to make sure the vehicle has a reasonable cruise rpm if it’s going to be driven on the street. We don’t want to be going down the street at 70 miles-per-hour with say a 4.11 gear on the vehicle and have a cruise rpm of say 3,200-3,500 RPM that will put way too much load on the engine and cause excess engine wear, poor fuel mileage, and things of that nature. We want to make sure it matches what we are doing with that vehicle and the vehicle use. Once again, all the last three of these kind of relate to one another.

Is this a drag race vehicle, is it a truck that is being driven off road, is it strictly street driven? We want to make sure that gear set is going to go ahead and match what we are going with that vehicle as end result. Here are some common ratios that can be used as a guide to determine what gearing is correct for your application. If we have a daily driver, 2.55 to 3.25 gear ratios typically work best, whereas a street/strip application we would usually use a 3.42 to a 3.90 gear ratio, and race applications most commonly use a 4.10 and above gear ratios. So what effect will different ratio gear sets have on a vehicle operation? Well, let’s say for example we had a vehicle we install 2.73 gears in it. 2.73 gears are considered to be a numerically low gear set and this type of gear set will give the vehicle slow or poor acceleration from dead stop but at the same time give us lower cruise rpm’s as well as top-end speed or top-end mile- per-hour. This makes it a good street gear or a good daily driven type gear for highway use or for a vehicle we are looking to get better fuel mileage out of. Just from a comfort standpoint, it will tax the vehicle less when it comes to engine rpm. In comparison, if we were to look at installing a set of 4.56 gears in that same vehicle, a 4.56 gear will give you way better acceleration from a dead stop but also go ahead and tax the motor much more when it comes to cruising speeds on the highway. What it will essentially do is lower the max mile-per-hour that the vehicle is able to achieve because it is making the engine reach its max rpm potential so much quicker in comparison to the 2.73 gear set. This makes a 4.56 gear used more commonly in off road and race applications, because the fact that in those applications, we want to really want to reach the max rpm of the engine much quicker, because we are trying to get the vehicle to accelerate at a much quicker rate of speed. The only times where these rules kind of don’t hold true is if we get into the situation where we have a vehicle with extremely tall tires.

An extremely tall tire will have a drastic effect on how that gear ratio responds in that vehicle. So let’s say we have a 40-inch tall tire. With that 40-inch tall tire, if we were to have that 4.56 gear,  that may make that 4.56 gear actually act like that 2.73 gear in that specific situation. In the end, that would make us really be even more a candidate for something like a 5.13 gear or a 5.88 gear to get our cruise rpm to the point where we want it to be. So just remember, we have to look at all of the aspects to the vehicle as a whole to determine what ratio is best for our application. Now that you have determined what ratio gear set is correct for your application.

There are three things you need to determine about the rear-end before your selection of that gear set. The first think you need to look at is the type of rear-end that is at the car or truck or whatever you may have. Certain vehicle applications have the availability of different rear-ends depending on the vehicle options or that rear-end could have been changed out in that vehicle at some point in time or another. So it is important for you to get underneath the vehicle take a look at the rear end to determine exactly what type of housing was used in your specific vehicle application to determine what gear set is correct for it. Sometimes the stalls can be determined when you have rear-ends that are similar to one another by measuring the outer diameter of the ring gear as well. There is a small difference in housing size between say a GM eight and a quarter compared to GM eight and a half rear end so a lot of times if you don’t know how to identify just be looking at it you have to take the inspection cover off measure the diameter ring gear and go forward from there.

The second thing you need to look at is if you have a GM or Dana axle. These use different series of gears. What that means is they use different thicknesses of ring gears depending on the type of carrier that what was used. Now if the vehicle already has the original gear set installed in it, what you can do is use that ratio of gear set that to determine what series it was according to the parameters of what the carry series was for that specific type of rear-end. You can find that type of rear-end a lot of times on our website listed with the gear sets or in the manufacturer catalogs if you have those available to you.

The third thing that may need to be determined is the type of gear that you are going to use for your application. There are basically two different versions of gears produced– there is a performance gear, also known as street gear a lot of times, and there is also a Pro Series gear. The performance slash street series gear is designed for applications where the vehicle is making more power than stock and you want a little stronger gear a little more performance oriented type gear in comparison to what the stock gear was. A Pro Series gear on the other hand is a much softer gear. A Pro Series gear is designed for drag race applications only. The reason the gears are softer is they are designed to absorb the impact between the sticky tires and the harsh impact of the drivetrain being transmitted through that gear set, so it doesn’t chip teeth and things of that nature. What would happen if you put a Pro gear in a street car is you wear that gear out very quickly and that is why they are designed for drag race applications only. So remember when looking for a gear, most likely you are going to need a performance or street gear, and Pro Series gears once again are for drag race use only. One last note about selecting a gear set for your rear-end is sometimes in some GM or Dana axle applications, where a carrier series is a  concern,  what you want to install on the rear end is not available for that series of carrier.

In some situations there is a solution to this problem, and that is what’s known as a ring gear spacer. A ring gear spacer is what’s designed to take up the extra space that will be created by having a series of gear that is higher than what is designed for your carrier because typically a two series gear will be much thicker than what a two series gear is. So what will happen is you will have this new thinner gear that is designed for a three-series carrier, and if you were to attach it to the two-series carrier what would happen is it would be too far away from the pinion gear, and the two would not attach to one another properly. What the spacer will do is it will go in between the carrier and the ring gear taking up that excess space making the  ring and pinion gears contact one another properly. When you purchase one of these, it will come with new longer bolts as well to go ahead and ensure that the bolts seat completely. These are good probably up to about 450 horsepower or below–anything past that you do not want to use a ring gear spacer. There is also a gear selection calculator on SummitRacing.com under the expert advice tab that will help you determine which gear ratio is best for your application.