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.
2x and 3x drivetrains
If your bike has more than one chainring, every chainring is paired with every cassette sprocket to build your full gear list, then sorted from fastest to easiest exactly as a 1x setup would be. A 2x11 drivetrain has 22 possible combinations; a 3x9 has 27.
Not every combination is one you'd actually use. Running your biggest chainring with your biggest cassette sprocket (or your smallest chainring with your smallest sprocket) puts the chain at a severe angle — this is called cross-chaining, and it wears your drivetrain faster without giving you a useful gear you don't already have elsewhere. These combinations are flagged and shown dimmed in the tables by default, with a toggle to reveal them if you want the complete picture.
Your top speed and easiest gear are always calculated correctly regardless of how many chainrings you have — they come from your fastest combination (biggest ring, smallest sprocket) and your easiest combination (smallest ring, biggest sprocket), neither of which is ever flagged as cross-chaining.
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 of crank arm movement, the bike moves 5 inches forward — a consistent way to compare gearing across bikes with different wheel sizes and crank lengths.
Jump %
Jump percentage measures how much your cadence drops when you shift to the next easier gear. It reflects what you actually feel when shifting — a smaller number means a smoother, more connected transition.
This is a ratio-based measure of actual cadence disruption, not just tooth count change. Small jumps are smooth and barely noticeable. Larger jumps — common at the upper end of wide-range cassettes — mean your cadence will change meaningfully when you shift. Wide-range cassettes (like 11-51t) typically have larger jumps near the bottom of the range as the tradeoff for having extreme bailout gears. The colour coding in the comparison table (green, orange, red) gives you an at-a-glance sense of which shifts are smooth and which are noticeable.
Sustained Grade — the climbing model
Sustained grade is calculated using a physics model that accounts for your power output, system weight, speed in each gear, and rolling resistance. It gives a practical estimate of the steepest grade you could sustain at a steady effort under consistent conditions — not a burst, and not a laboratory-grade prediction.
Rolling resistance ≈ a standard coefficient for mixed terrain × system weight
Climbing force = available force − rolling resistance
Sustained grade = climbing force ÷ system weight
The model accounts for typical drivetrain friction losses from a clean, well-maintained chain and cassette, and a rolling resistance value appropriate for mixed road and trail surfaces.
Aerodynamic drag is excluded because at climbing speeds (typically under 10 mph) it contributes a small fraction 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.