When we talk about gearing, we’re referring to the final-drive ratio, which you get by dividing the number of teeth on the rear sprocket by the number of teeth on the front, or countershaft sprocket. This figure represents the number of times the front sprocket has to rotate to turn the rear sprocket, and ultimately, it determines how engine rpm translates to road speed and how much torque there is at the rear tire.
Before we get into the nuts and bolts of it, let’s talk lingo. When someone says that a bike is geared “tall” or “high,” that means you’ll get a higher top speed but at the cost of acceleration. If someone says that gearing is “short” or “low,” that means better acceleration but a lower top speed. Here’s the confusing part though. A larger gear ratio—say, 3.10—correlates to shorter gearing, while a low number—like 2.70—represents taller gearing. It all goes back to how many times that front sprocket has to rotate to turn the rear sprocket and the wheel.
The good news is that while crunching numbers to compare ratios is helpful, it’s not necessary. All you really need to know is what your bike’s current gearing is—it’s printed right on the side of the sprockets—and then figure out how you’ll need to adjust the tooth count to change your bike’s performance.
If you want better off-the-line acceleration, say, because you ride a small-displacement bike or predominantly ride in the city or really tight canyons, you’ll want to lower your gearing by either subtracting teeth from the front sprocket or adding them to the rear sprocket. If you want to get better gas mileage and net lower rpm as you cruise down the freeway, you’ll want to either add teeth to the front or subtract them from the rear.
For the most part, riders tend to gear their bikes shorter since many motorcycles today come with excessively tall gearing. But even if a bike feels like the gearing is way off for the way you ride, you probably won’t have to change the tooth count much to notice a big difference. In general, people will change the front sprocket by one, maybe two teeth, and alter the rear by perhaps two or three teeth at maximum.
If you change your gearing, even by just a tooth, you’re changing the diameter of the sprocket and it’s going to affect your chain slack, so make sure you readjust the chain tension. Most minor gearing changes of just a tooth or two can be accounted for with the available adjustment at the swingarm, but you don’t want to mess with your wheelbase too much, and if you’re going big on the rear, you may find that you need a longer chain. Also, if you’ve got a decent amount of mileage on your drivetrain, you’ll want to replace everything as a set so it all wears evenly. Throwing a new sprocket at a worn chain is going to chew it up in a hurry.
Another thing to consider when changing your gearing is how it might affect your speedometer. Most bikes register speed off the transmission, so altering the final-drive gearing is going to screw up the math the ECU is doing. If you have an older bike with a front-wheel speed sensor you’re in the clear, but otherwise you might want to consider getting a SpeedoHealer or a similar calibration device.
Now, if you’re on a budget or just want to experiment to see how a gearing change affects your bike, the cheapest and easiest thing to do is to swap out the front sprocket. Front sprockets are cheaper, for one, at about $20 or $30, they’re held in place with less hardware, and a change of just a single tooth alters the ratio more than a single tooth on the back wheel. Rear sprockets cost more, and you have to remove the rear wheel to replace ’em.
Finally, when it comes to buying replacement sprockets you can go the OE route which will likely be a stamped-steel affair, or you can turn to the aftermarket where there’s a variety of aluminum and steel options. Aluminum is much lighter but also less durable, so might not be the best option for street riders who log a lot of miles. That’s where a hybrid sprocket like this SuperSprox comes in; it mates a long-lasting steel chain ring to a lightweight aluminum carrier so you get less mass and good durability. And, If your bike has a belt drive or shaft drive, you’re pretty much stuck with the gearing you’ve got. Changing the final-drive ratio on those bikes isn’t impossible but it is difficult and expensive. Meanwhile, you can replace the entire drivetrain on your chain-driven bike in about an hour and for just a couple hundred bucks.