Even if your car has a healthy engine and tires, you may find that getting up to speed on the freeway take effort. More often than not, it’s due to the gear ratios of its drivetrain, i.e., how the number of teeth of two interlocking gears relate to one another, which dictates what your car prioritizes: speed vs. Force. Learning about this trade-off will improve your driving, modification, and purchase decisions.
Why do moddern cars have so many gears? Why not just a six speed? Because the ratios is different. Here’s the math: This goes back to that whole overdrive thing where the output shaft (wheels) are spinning faster then the input shaft (engine). It sounds odd. Why would you want your wheels to turn faster than your motor? Well, because it reduces engine RPM at highway speeds. When RPMs is lower, the engine doesn’t have to work so hard to keep up its speed. This reduces vibration for a quiet ride in cab and saves gas to improve fuel economy. And though you typically won’t feel these small improvements on the road, collectively the difference add up: hundreds of gallons saved per year.
How Gears Help Your Car
Until there’s an issue, many vehicle owners fail to consider their rear axle ratio. For rock crawling or towing a large load, a high numerical ratio such as 4.56 increase torque significantly. While that’s great for off-road use, it also cause the engine to rev fast while driving on the highway. Each mile result in lost gas mileage and you’ll hear your truck whine. On the other hand, a 2.73 ratio is tall, so it keep the RPMs low when cruising but makes it hard to accelerate away from a complete stop. For most everyday driving, people strike a happy medium with a 3.5ish. This provide good acceleration and decent highway performance.
When installing bigger tires without changing gears, you made a common mistake. What happens here is that by putting 35-inch tires on a truck equipped with 28-inch tires, you’ve now decreased your overall gear ratio. This means you have taller axles without realizing it. Now the engine has to spin more times to make the tire turn one time, that results in poor fuel economy and sluggish take-off. This can be easily fixed by changing the differential gears and replacing them with numerically higher gears to return the lost multiplication. This is a simple correction but if ignored, you’ll end up with a capable truck that becomes a slug. You should of changed it sooner.
It’s the same physics dilemma for cyclists and their bikes. If you have a big ring in front and a little one on the back it gives you speed but can make every hill feel like a mountain. Swap to a bigger cog or smaller ring and you reverse this equation and trade speed for climbing skill. On any machine the same rule applies; the gearing dictates how much leverage you have over resistance. Whether you’re pulling a trailer or climbing a high percentage grade, using proper gearing match power to the task at hand. It is not about sheer horsepower, but about using that horsepower in an efficient way to get it onto the ground. Get the math correct and it becomes easy.
