Using online gearing top speed calculators I realized that the bigger the diameter of the wheels, the higher the theoretical gearing top speed is. Why is that? Why does bigger wheel act as increasing the gear ratio and thus increasing the theoretical gearing top speed, does bigger wheels also reduce the torque going to the ground because it makes the gearing taller?
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1You use the word "wheel" in both the title and the body text. Wheel size isn't determinative. Instead, it's the size (diameter and therefore circumference) of the tire that affects the car's gearing. Because tires can have different aspect ratios, same-size wheels with different aspect ratio tires will result in changed gearing. The overall tire diameter is what matters.– DavidRecallsMonicaOct 31, 2022 at 3:12
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yes thats what i meant with wheel. doesnt wheel mean both rim + tire combined?– Timothy SchererOct 31, 2022 at 14:17
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I agree that the word "wheel" is sometimes used to indicate the wheel + tire combination, but that use is imprecise because of "sometimes." Your question addresses the exterior dimensions of the tire; because you don't define what you mean by the word "wheel," you risk the reader getting off track. It would be more accurate here to use "tire."– DavidRecallsMonicaOct 31, 2022 at 14:23
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1 Answer
The answer lies in the rpm conversion to ground travel. Any gearing in a motor vehicle is designed to provide sufficient torque to begin moving (with load, of course) and to keep the engine in the appropriate rpm range.
Consider one thousand engine revolutions. The transmission converts that through the cogs and shafts and belts (CVT) and torque converters (automatic transmissions/locking) into a different value on the output shaft. Depending on the vehicle design, this revolution is then passed through a differential, which is going to change the value again.
Allowing for the incredibly high revolutions per minute of an internal combustion engine, the ratio will be substantial. As an example, Car & Driver magazine, November 2022, p.53, lists 5.2 miles per hour per 1000 rpm for the 2023 Toyota GR Corolla Circuit. This would represent the speed on the tested tires, Michelin Pilot Sport 4, 235/40ZR18. Sixty thousand revolutions during that hour traveling five point two miles calculates to about six inches for each engine revolution. An increase in tire diameter of approximately one-third of an inch adds an inch of travel.
Changing the tire size to something smaller would result in less distance traveled during that one hour and conversely, a larger tire would result in a greater distance traveled.
As you suggest, the effective gearing is also changed. The last mechanical conversion of the power train is the distance from the center of the wheel to the ground. A lever with a greater length (larger tire), pivoting from one end (wheel hub) requires more force to move the equivalent mass at the end (entire motor vehicle) than a smaller lever (smaller tire) does.
Small tires = more torque, less speed. Large tires = less torque, more speed.
This is obviously a simplification, but does factor in when one allows for the necessity to either re-calibrate one's speedometer or determine the speed change with a tire change.
answered Oct 29, 2022 at 21:12
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Actually, small tires don't increase torque compared to large tires. They increase force. With a small tire, the torque is the same but the force that torque creates is larger. So the tire is the device that converts rotational movement and force (torque) to linear movement and linear force.– juhistOct 30, 2022 at 12:23
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@juhist - What fred is stating about small tires and the increase in torque is not inaccurate, just simplistic. A smaller tire does increase the torque applied (ie: force) in changing the twisting motion to linear force. The Khan Academy states, "An increase in torque comes with a proportional decrease in rotational speed.", which is pretty much what fred is stating. The tire is a cog in the system between the engine and the road. Reducing radius increases the applied torque.– Pᴀᴜʟsᴛᴇʀ2 ♦Oct 31, 2022 at 12:23