This question reveals a very subtle but crucial difference in riding feel: "mechanical gear ratio" vs. "perceived gear ratio."

The answer is: Tire width doesn't directly change mechanical gear ratio, but it does change the "final drive ratio" and your power delivery feel, so riding feels very different.

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1. Mechanical Gear Ratio vs. Final Drive Ratio

First, we need to clarify two concepts:

• Mechanical Gear Ratio: This is purely the mathematical ratio between chainring teeth and cassette teeth. For example, a 50T chainring with an 11T cog gives a gear ratio of 50 / 11 ≈ 4.54. This ratio is fixed and unchanging, regardless of what tires you use.

• Final Drive Ratio / Development: This is the "Development (gear meters)" concept we've discussed before, which calculates the actual distance the bike travels per pedal revolution. Its formula is: Mechanical Gear Ratio × Wheel Circumference.

Here's the key point: Even just changing from 25mm to 32mm, wider tires also increase in overall height when inflated, thus causing the total wheel circumference to become larger.

Calculation Example: Assuming on a 700c rim:

• Using 25mm tires, total circumference might be 2136mm.
• Changing to 32mm tires, total circumference might increase to 2174mm.

Now, let's see the difference in the same gear (e.g., 50/17T, gear ratio 2.94):

• 25mm tire: 2.94 × 2.136m = 6.28m (travels 6.28 meters per pedal revolution)
• 32mm tire: 2.94 × 2.174m = 6.39m (travels 6.39 meters per pedal revolution)

Conclusion: After changing to 32mm tires, in the same gear, you travel 11 centimeters further per pedal revolution. This is equivalent to your gear ratio being "secretly" made slightly heavier.

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2. Why Do 25mm and 32mm Feel So Different?

The difference you feel is the result of the above "final drive ratio" change, plus the more important "rolling resistance" and "comfort" working together.

Rolling Resistance

In the past, people believed "narrower, higher-pressure tires roll faster," but modern scientific research has proven that on real, imperfect paved roads, this view is wrong.

• 25mm tires: Require very high pressure (e.g., 90-100 PSI). On uneven roads, they're like a hard ball—when hitting small bumps, they bounce upward, causing vertical energy loss and increasing rolling resistance. The sensation is "bumpy" and "clear road feel."
• 32mm tires: Can use lower pressure (e.g., 60-70 PSI). They have larger air volume, and when hitting bumps, the tire itself deforms and absorbs vibration rather than bouncing the entire bike and rider upward. This reduces vertical energy loss, so rolling resistance on real roads is actually lower. The sensation is "smooth" and "stable."

Comfort and Power Delivery Feel

• 25mm tires: High-frequency vibrations from the road continuously impact your body, causing faster muscle fatigue. To maintain speed, you need to constantly fight these vibrations.
• 32mm tires: Greatly filter road vibrations. Your riding is more stable, body more relaxed, and core strength can be used more purely for pedaling output rather than stabilizing the bike. This makes maintaining power output over long periods subjectively easier.

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Summary

Characteristic	25mm Tires	32mm Tires
Mechanical Gear Ratio	No change	No change
Final Drive Ratio	Slightly lower (smaller wheel circumference)	Slightly higher (larger wheel circumference, gears feel slightly heavier)
Rolling Resistance (real roads)	Higher	Lower (smoother)
Comfort	Poor, bumpy	Good, strong vibration absorption
Subjective Feel	"Hard," "fast," "bumpy," "clear road feel"	"Stable," "smooth," "comfortable," "feels more effortless"

Ultimately, the "difference" you feel is a compound experience:

After changing to 32mm tires, although your gears became objectively slightly heavier due to the larger wheel circumference, thanks to reduced rolling resistance and greatly improved comfort, your pedaling efficiency and power delivery feel actually became better, ultimately feeling "easier" and "smoother."