38×17 gearing on 700×25c with 170 mm cranks
| Gear inches(38 ÷ 17) × 26.3″ wheel | 58.8in |
|---|---|
| Gain ratiothe only one of the three that accounts for 170 mm cranks | 4.39 |
| Developmentground covered per full pedal revolution | 4.69m |
| Gear ratio38 teeth ÷ 17 teeth | 2.24: 1 |
| Speed at 90 rpm25.3 km/h | 15.7mph |
| Wheel diameter used700 × 25c — outside of the tyre, not the rim | 26.3in |
Notes
- Gain ratio is the number worth learning. Gear inches came from penny-farthings and pretends every bike has the same crank length; 170 mm cranks change the leverage at the pedal, and only gain ratio shows it. A gain ratio near 2.5 is a comfortable flat cruising gear for most riders, and anything under 2.0 is climbing territory.
- Tyre width is in this number. 700 × 25c rolls out at 26.3″, and swapping to a much fatter tyre on the same rim moves the effective gear by more than a sprocket tooth. Calculators that ask only for rim size are throwing that away.
- 15.7 mph assumes 90 rpm, which is a steady road cadence rather than a maximum. Divide or multiply straight through for your own cadence — the relationship is linear.
- Chainring and sprocket wear together. A worn chain on a worn cassette can skip under load in exactly this gear while shifting fine in every other one, which is worth checking before blaming the derailleur.
Three numbers for the same gear
58.8 gear inches. The old imperial measure — what size penny-farthing wheel would give this gearing. Universally quoted, and it quietly assumes every bike has the same cranks.
4.39 gain ratio. Sheldon Brown's measure, and the only one of the three that knows your cranks are 170 mm. Two bikes with identical gear inches and different crank lengths do not feel the same at the pedal, and gain ratio is what shows it. Around 2.5 is comfortable flat cruising; under 2.0 is climbing.
4.69 m of development. Ground covered per complete pedal revolution — the measure most of Europe uses.
Tire size is in here, and it should be
700×25c rolls out at 26.3″ measured over the tire, not at the rim. Swapping to a much fatter tire on the same wheel moves the effective gear by more than a sprocket tooth. A gear calculator that asks only for wheel size is throwing that away.
At the pedals
15.7 mph at 90 rpm (25.3 km/h). Ninety is a steady road cadence, not a maximum — the relationship is linear, so scale straight through for your own.
One thing to check first
A worn chain on a worn cassette can skip under load in one specific gear while shifting perfectly in every other one. If this is the gear that slips, the derailleur is probably innocent.
Nearby sizes
- 36×17 gearing on 700×25c with 170 mm cranks
- 40×17 gearing on 700×25c with 170 mm cranks
- 34×17 gearing on 700×25c with 170 mm cranks
- 42×17 gearing on 700×25c with 170 mm cranks
- 38×16 gearing on 700×25c with 170 mm cranks
- 38×18 gearing on 700×25c with 170 mm cranks
- 38×15 gearing on 700×25c with 170 mm cranks
- 38×19 gearing on 700×25c with 170 mm cranks
- 38×17 gearing on 700×32c with 170 mm cranks
- 38×17 gearing on 700×40c with 170 mm cranks
Read more
- Gear Inches and Development — Two Ways to Compare Any Two BikesA 34/28 on one bike and a 30/25 on another are almost the same gear. Ratios alone will not tell you that; gear inches will.
- What Cadence Should You Ride? The One You Already Choose, MostlyRiders self-select a cadence close to their own metabolic optimum without being told. Which makes the useful question not what cadence, but whether your gears let you hold it.
- Gearing for Steep Hills: The Lowest Gear Is the Only One That MattersNobody runs out of top gear going uphill. Choosing gearing for hills is entirely a question about the other end of the cassette.
- Bike Gear Ratios: Gain Ratio Is the Only Number That Compares Two BikesFour ways to express a gear and only one accounts for crank length. Compare two bikes with the wrong one and the answer is quietly wrong.