How the numbers work
Speed
Speed is calculated from your gear ratio, cadence, and wheel size. The gear ratio is how many times the rear wheel rotates per pedal stroke (chainring teeth ÷ cassette sprocket teeth). Multiply that by your cadence and your wheel's rolling circumference and you get forward speed.
Wheel circumference is calculated from the nominal diameter of your wheel and tire combination. Actual rolling diameter varies slightly with tire pressure, rim width, and rider weight — the numbers shown are accurate to within about 1-2% for typical setups.
Gear Inches
Gear inches is the oldest way to compare bicycle gearing, dating to the penny-farthing era. It tells you the diameter a direct-drive wheel would need to produce the same gearing as your setup. Bigger number means a harder gear.
It's the standard comparison unit in the US. Its limitation is that it doesn't account for crank arm length — which is why gain ratio exists.
Development (meters)
Development is how far your bike travels in one complete pedal stroke. More useful than gear inches when comparing bikes with different wheel sizes, since it gives you the actual distance moved rather than an abstract diameter.
Gain Ratio
Gain ratio is Sheldon Brown's improvement on gear inches. It answers the question: for every inch my foot travels in a circle, how far does the bike move forward? Because it factors in crank arm length, it's the only metric that lets you fairly compare gearing between bikes with different wheel sizes and crank lengths.
Both wheel radius and crank arm length are in the same unit (meters), making the result dimensionless. A gain ratio of 5.0 means for every inch your foot moves, the bike moves 5 inches forward.
Jump %
Jump percentage is the tooth count increase from one sprocket to the next, expressed as a percentage. It measures how abrupt the cadence change is when you shift. Smaller jumps mean smoother, more connected shifts.
Under 18% is generally smooth and barely noticeable. Around 18-28% is noticeable but manageable. Above 28% is a large step — common in wide-range cassettes at the upper end — where your cadence will change meaningfully when you shift. Wide-range cassettes (like 11-50t) typically have large jumps in the middle of the range as the tradeoff for having extreme gears at both ends.
Max Grade — the climbing model
Max grade is calculated using a physics model that accounts for your power output, system weight, speed in each gear, and rolling resistance. It represents the steepest grade you could realistically sustain at a steady effort — not a burst, and not a theoretical mechanical maximum.
Rolling resistance (N) = 0.005 × system mass (kg) × 9.807
Climbing force (N) = available force − rolling resistance
Max grade (%) = (climbing force ÷ (system mass × 9.807)) × 100
The 0.97 factor is drivetrain efficiency — a clean, well-lubricated chain and cassette typically loses about 3% of your input power to friction. The 0.005 rolling resistance coefficient is a reasonable middle ground for mixed surfaces.
Aerodynamic drag is excluded because at climbing speeds (typically under 10 mph) it contributes less than 1% of total resistance. Including it would add complexity without meaningfully changing the results.
Setting the power slider
The power slider affects every grade calculation. Set it to match your typical sustained climbing effort — not a sprint, but the power you can hold for several minutes on a real climb.
| Rider type | Typical sustained watts |
|---|---|
| Casual / recreational | 100–150 W |
| Regular rider, decent fitness | 150–220 W |
| Fit enthusiast with structured training | 220–300 W |
| Competitive amateur | 300–400 W |
If the grade numbers look too high or too low for what you actually experience on your rides, adjust the power slider until they match reality. That calibration makes every comparison more accurate for your specific situation.
Accuracy and limitations
Tire diameter is based on nominal sizing. Actual rolling diameter varies by rim width (wider rims make tires sit taller), tire pressure, and casing construction. The error is typically 1-2% which translates to a similar error in speed and development figures.
The climbing grade model assumes constant power on a smooth, uniform gradient with no wind. Real-world climbing involves variable effort, surface texture, and headwind — all of which affect actual climbable grade. The numbers are accurate as a comparison baseline even if the absolute values differ from what you'll experience on a specific hill.