Choosing Rear Gears 101 With Silver Sport Transmissions


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For many modifiers and hot rodders, gearing is often an afterthought. Sometimes, they are going for ultimate horsepower or ultimate handling or ultimate car show appeal. But sometimes if you want your ride to give you a bigger kick in the pants then it pays to pay attention to your rear. Differential that is. That’s where Silver Sport Transmissions comes in. They can get you off the line in a hurry and with a bit of lingo tutoring you can get with them on what you need. Here we go:

Gear Ratio: In your car’s rearend there is a completely round ring gear and an accompanying pinion gear that rides around on that ring gear. If you take the number of teeth on one of these gears and the teeth on the other you get your gear ratio or “final drive ratio”. Let’s you have 4.10:1 gears (“4.10s” is the street talk version).

That means your pinion gear will turn 4.10 times for every single turn of your ring gear. Put another way – that you can actually see under the car -for every 4.10 turns of the driveshaft, the rear wheel will spin once. Silver Sport has an explainer on how to determine your current gear ratio HERE.

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Short Gear Sets: The higher the numbers are (4.30, 4.11, 4.56, etc.) the shorter the gears. Yes that is counter-intuitive, we know. Tall gears should have tall numbers right? And short gears should have smaller numbers? You’d think, yes, but that’s not the way it works with gearsets. When we say “short gears” we mean gears made for quicker off-the-line acceleration. But they aren’t very good for cruising on the interstate because the engine must maintain a very high RPM to stay at the higher speeds. So your stereo will definitely be drowned out by your screaming engine if you want to maintain 75mph with a 4.56 gearset out on the freeway.

Tall Gear Sets: The lower the numbers are (3.25, 3.00, 2.90, etc.) the taller the gears. These are the ones you need for long-haul, high-speed cruising. Not so great for the stop-light Grand Prix or bracket racing at your local strip. We are talking about achieving top end speed here.

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Even without changing your gears there are things that affect final drive ratio, for instance:

Your rear tire height: A tire 21” tall has a 66” circumference. (21” x π). So one revolution of the tiremoves the car the car 66”. Taller tires travel greater distances for the same amount of drivetrain rotation. Tall tires can take a car with a 4.56 gearset and make it feel like it only has 3.90s or 3.73s.

Your transmission’s internal gear ratios: In transferring the power of the rotating engine along the driveshaft to the rear wheels, a lot can happen. And much of it happens inside your transmission. As an example, the TKO 500 tranny that you can get from Silver Sport Transmissions has a first gear with a 3.27 ratio. This means the engine turns 3.27 times and the transmission spins the driveshaft once. In first gear, your engine may be churning at 3000 RPM, but because of the loss of rotation (which, again, is happening inside the transmission), the car only travels about 14mph.

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On the other hand, the TKO’s overdrive gear is .68. Remember our tall and short discussion about gearsets? It applies here, inside the trans as well. Because at that same 3000 RPM, in overdrive, the transmission spins the driveshaft one turn every .68 turns of the engine and the car will travel at 67.23 MPH. That’s a nice highway speed and you can still hear the stereo (we are assuming a car with 4.10 axle and a 21” tire for the sake of this discussion). An overdrive transmission can lessen the RPM load the engine must deliver to the rear axle, which will allow you to run a shorter rear axle (great for acceleration) without sacrificing your top speed – best of both worlds. That’s why the world loves overdrives. And Silver Sport’s got ‘em.

Think about how you’ll be using your car and how it’s driving now. The technical representatives at Silver Sport Transmissions and their speed analyzer can help you figure out how different gear sets, transmissions, and tire heights will affect your speed and engine RPMs. With what you learn, you can determine the right gears for you, whether you want a drag strip terror, a top-end tiger, or a happy medium in between.

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Des Ryan

Des Ryan has been involved in all levels of Mopar musclecar ownership, from barn finds to drivers to a high-level restored Hemicar. He's also spent over 20 years in the advertising industry as a copywriter and creative director. With pen in one hand and pistol-grip shifter in the other, Des is bringing his two passions together to keep you Direct-Connected.

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