How much power does your drivetrain lose?

The short answer: a clean, oiled derailleur drivetrain gets about 98% of your pedal power to the rear wheel when you push hard. At low power, and in the wrong gear, it loses more. FondoLab assumes a 2.5% loss unless you change it.

What the lab measured

Two researchers drove eleven bicycle transmissions with a motor and measured the power going in and coming out, at 50 to 370 watts, with the chains well oiled ¹. What they found:

• Efficiency rises with power. A derailleur drive went from about 91% at 50 W to over 97% at 370 W as measured. Corrected for the test rig’s own losses, it was over 98% at the highest load.
• Gear choice matters. Neighbouring gears often differed by 1–3% in efficiency, up to 4% in the 27-speed drive tested. Picking a poor gear can easily cost about 2%.
• Bigger gear ratios were a little less efficient on average.
• Hub gears were generally about 2% less efficient than derailleur gears, with exceptions.

What it means on a fondo

At 200 watts, every percent of drivetrain loss is 2 watts that never reach the road. Over a long day that adds up, and unlike drag area or tyre pressure, some of it is free to win back: keep the chain clean and oiled, as the tested chains were.

The FondoLab default

The rider profile starts every bike at a 2.5% drivetrain loss. That is a FondoLab default, not a number from the study: the lab’s best case was under 2%, at high power with clean chains, and a real ride mixes lower power, every gear and a chain that is not always fresh. If you have a reason to use another number, such as a hub gear, change it per bike. The source is on the method page.