Bike Gear Inch Calculator
Calculate your bicycle's gear inches to understand gear ratios and find your optimal setup.
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Complete Guide to Bicycle Gear Inches and Ratio Calculations
Understanding gear inches is essential for optimizing your bicycle's gearing for different terrains and riding styles. This comprehensive guide explains everything you need to know about gear ratios and their impact on cycling performance.
What Are Gear Inches?
Gear inches represent the equivalent diameter of the drive wheel if the bicycle were using direct pedaling without gearing. It's calculated as:
Gear Inches = (Chainring Teeth ÷ Cog Teeth) × Wheel Diameter
This measurement allows cyclists to compare gearing across different wheel sizes and drivetrain configurations.
Gear Inch Ranges by Bike Type
| Bike Type | Low Gear | High Gear | Range | Typical Setup |
|---|---|---|---|---|
| Road Bike | 30-35" | 110-125" | 350-400% | 50/34T with 11-28T |
| Gravel Bike | 20-25" | 100-110" | 400-500% | 46/30T with 11-34T |
| Mountain Bike | 18-22" | 90-100" | 450-550% | 32T with 11-50T |
| Hybrid Bike | 25-30" | 90-100" | 300-350% | 48/36/26T with 11-32T |
| Time Trial | 35-40" | 120-130" | 300-350% | 54/42T with 11-25T |
Gear Inch Interpretation Guide
| Gear Inches | Terrain Suitability | Cadence at 20mph | Perceived Effort | Typical Use |
|---|---|---|---|---|
| 15-25" | Extreme climbing | 120+ RPM | Very Easy | Steep MTB trails |
| 25-35" | Steep hills | 100-120 RPM | Easy | 10%+ grades |
| 35-50" | Moderate hills | 85-100 RPM | Moderate | 5-10% grades |
| 50-70" | Flat terrain | 70-85 RPM | Firm | General riding |
| 70-90" | Fast flats | 60-70 RPM | Hard | Group rides |
| 90-110" | Descending | 50-60 RPM | Very Hard | Sprinting |
| 110-130" | High speed | 40-50 RPM | Maximum | Time trials |
Gain Ratio Calculation
Gain ratio provides another way to understand gearing by considering crank length:
Gain Ratio = (Wheel Radius ÷ Crank Length) × (Chainring Teeth ÷ Cog Teeth)
This measurement accounts for the mechanical advantage based on your pedal stroke.
| Gain Ratio | Pedaling Feel | Optimal Cadence | Best For |
|---|---|---|---|
| 3.0-4.0 | Very Light | 90-110 RPM | Climbing |
| 4.0-5.0 | Light | 85-95 RPM | Hill training |
| 5.0-6.0 | Moderate | 80-90 RPM | General riding |
| 6.0-7.0 | Firm | 75-85 RPM | Flat terrain |
| 7.0-8.5 | Hard | 65-75 RPM | Speed work |
| 8.5+ | Very Hard | 50-65 RPM | Sprinting |
Wheel Size and Tire Width Impact
| Wheel Configuration | Circumference | Gear Inch Change | Speed Impact |
|---|---|---|---|
| 700×23mm | 2096mm | Base | Reference |
| 700×28mm | 2136mm | +2% | +0.5km/h at 90RPM |
| 650b×47mm | 2055mm | -2% | -0.5km/h at 90RPM |
| 29×2.1" | 2280mm | +9% | +2km/h at 90RPM |
| 26×2.3" | 2050mm | -2.2% | -0.6km/h at 90RPM |
Optimal Gear Selection by Terrain
| Terrain | Ideal Gear Inches | Chainring:Cog | Cadence | Notes |
|---|---|---|---|---|
| Steep climbs (10%+) | 20-30" | 34:32 or 30:34 | 80-95 RPM | Lowest gears |
| Moderate hills (5-10%) | 30-45" | 34:28 or 39:25 | 75-90 RPM | Comfortable spinning |
| Rolling terrain | 45-65" | 50:19 or 52:17 | 80-95 RPM | Versatile range |
| Flat roads | 65-85" | 50:14 or 53:15 | 85-100 RPM | Efficient cruising |
| Fast flats/descents | 85-110" | 50:11 or 53:12 | 70-85 RPM | Speed maintenance |
Rider Level and Gear Selection
| Rider Level | Preferred Range | Low Gear | High Gear | Cadence |
|---|---|---|---|---|
| Beginner | 25-85" | 34:32 | 50:17 | 70-85 RPM |
| Recreational | 30-95" | 34:28 | 50:15 | 75-90 RPM |
| Enthusiast | 35-105" | 39:28 | 52:14 | 80-95 RPM |
| Racer | 40-120" | 39:25 | 53:12 | 85-100 RPM |
| Pro | 45-130" | 53:28 | 55:11 | 90-110 RPM |
Frequently Asked Questions About Gear Inches
General Questions
What is a good gear inch range for road cycling?
For general road cycling, most riders benefit from:
- Low gear: 30-35" (for climbing)
- High gear: 100-110" (for descents)
- Sweet spot: 50-70" (flat terrain)
This typically requires a compact chainset (50/34T) with an 11-28T cassette.
How do I calculate speed from gear inches?
Use this formula:
Speed (mph) = (Gear Inches × π × Cadence × 60) / 1056
Example: 100 gear inches at 90 RPM:
(100 × 3.1416 × 90 × 60) / 1056 ≈ 25.7 mph
Technical Questions
What's the difference between gear inches and meters development?
Key differences:
| Metric | Calculation | Units | Usage |
|---|---|---|---|
| Gear Inches | (Chainring ÷ Cog) × Wheel Diameter | Inches | North America |
| Meters Development | (Chainring ÷ Cog) × Wheel Circumference | Meters | Europe |
100 gear inches ≈ 8.0 meters development
How does crank length affect gearing?
Crank length impacts:
- Leverage: Longer cranks provide more torque
- Pedaling circle: Affects optimal cadence
- Gain ratio: Incorporated into calculation
Standard lengths:
- 165-170mm: Small riders
- 172.5mm: Average males
- 175mm: Tall riders
Practical Questions
How do I choose the right gears for hilly terrain?
For hilly riding, consider:
- Lowest gear of 25-30 gear inches
- Compact or sub-compact chainset (50/34T or 46/30T)
- Wide-range cassette (11-34T or 11-36T)
- Close ratios in middle gears
- Test on local hills before committing
Should I prioritize gear range or close ratios?
It depends on your riding:
| Priority | Best For | Setup | Example |
|---|---|---|---|
| Wide Range | Hilly terrain | 11-34T cassette | 46/30T with 11-34T |
| Close Ratios | Flat terrain | 11-25T cassette | 53/39T with 11-25T |
| Balanced | Mixed riding | 11-28T cassette | 50/34T with 11-28T |