“Is 34x32 low enough?” is unanswerable until you put both bikes on the same scale. Gear inches does that: teeth ratio times wheel diameter, one number that lets a customer compare a gravel bike to the road bike already in the garage.
50x11 on a 700x25c wheel is 119.5 gear inches; 34x32 on the same wheel is 27.9. That is the whole range in two numbers.
The example
Three gear combinations on the same 700c wheel, which measures about 26.3 inches across including the tire.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Setup | Chainring | Cog | Wheel dia in | Gear inches |
| 2 | Top gear | 50 | 11 | 26.3 | 119.5 |
| 3 | Climbing gear | 34 | 32 | 26.3 | 27.9 |
| 4 | Single speed | 42 | 18 | 26.3 | 61.4 |
The formula
One multiplication, one division, one rounding:
How it works
What each piece means on the bike:
B2/C2is the drivetrain ratio — how many wheel turns you get per crank turn. 50/11 is 4.55; 34/32 is 1.06.*D2scales the ratio by the wheel. This is why a 29er and a 26-inch bike with identical cassettes do not feel identical.- Wheel diameter is measured over the inflated tire, not the rim. A 700x25c is about 26.3 inches; a 700x40c gravel tire is closer to 27.6.
ROUND(...,1)keeps it readable. Nobody needs the third decimal of a gear inch.
Build the column once for the bike a customer owns and once for the bike on the floor, and the fit conversation stops being about brand names.
Try it: interactive demo
Enter the chainring teeth, cog teeth, and the wheel diameter over the tire.
Variations
Metres of development
Multiply gear inches by pi and convert to metres for the European scale.
Speed at a given cadence
Gear inches times pi times rpm, converted to miles per hour.
Pitfalls & errors
Wheel diameter is the tyre's outside diameter, not the rim size printed on the sticker. Using 27 for every 700c wheel throws every comparison off by two to five percent.
For a full cassette, put the chainrings across the top row and the cogs down the side, then use one formula with mixed references to fill the whole grid.
Do not compare gear inches across wheel sizes by ratio alone. 34x32 on a 29er is a meaningfully taller gear than 34x32 on a 26-inch bike, and that is exactly what this column exists to show.
Practice workbook
Frequently asked questions
What is a good low gear for a loaded touring bike?
Does this work for internally geared hubs?
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