Speed and power
State any one of speed, power or a target time and read the other two, on a slope you choose and for a rider you describe. It is the same physics the ride comparison runs on, with the course typed in rather than measured. One input decides most of the answer and nobody knows their own: your drag area. So the answer comes with the band that not knowing it puts around everything else.
166 W holds 30 km/h on the flat, and 40 km takes 1:20:00.
Give or take 7 W, because a drag area picked from a list is only good to about ±0.02 m². Everything else here you stated; this one you guessed, and on this road it is the figure the answer turns on.
Air 77%Rolling 23%
The bigger the air's share, the more a position or a wheel is worth. Raise the slope and watch weight take over.
Each one measured by riding this same distance again with that one thing changed and the power held. The course is half the answer: a kilogram is minutes on a pass and seconds on a flat road, which is why this is asked of the road above rather than in the abstract.
| Change | Over 40 km | What it is |
|---|---|---|
| 10 W more | −1:49 | held for the whole distance, not a sprint at the end |
| 1 kg lighter | −5 s | you or the bike; on the flat it barely shows |
| 0.01 m² less drag | −42 s | about what race-fit kit is worth over loose kit |
| 0.001 less rolling | −79 s | about one step of tyre |
Steady riding on a constant slope: one speed the whole way, no corners, no braking and no traffic. A real ride is none of those, so read this as what the road costs, not as a time you will do. Air density comes from the altitude and temperature above; the drivetrain is taken to return 97.6% of what you put in. If you want this answered from rides you actually did, with the drag area measured instead of guessed, that is what the compare tool is for.
For a 75 kg rider on a 9 kg road bike, on the hoods, on the flat and in still air: about 166 W, give or take 7 W. The doubt is almost all drag area — the one number nobody knows about themselves. Change the position, the weight or the wind above and watch how far it moves.
Because one input decides most of it and nobody knows their own: your drag area. The page carries that doubt through to the answer instead of hiding it, so you get the span a realistic spread of drag areas produces. On a steep climb the band nearly closes, because there weight does the work instead.
On the flat, much less than you would hope — speed goes up roughly as the cube root of power, so 20 W on 200 W is about 3% faster. On a climb it is nearly proportional, because you are lifting weight rather than pushing air. State a slope above and the page shows what one change is worth there.
Yes, as a headwind you state. A tailwind is a negative one. It is the same physics the ride comparison runs on, with the course typed in rather than measured from a file.
CdA, in square metres: how big you are to the air, times how badly you slip through it. The figures this site uses are 0.35 on the hoods, 0.325 in the drops and 0.23 on time-trial extensions. It is the single largest thing separating two riders at the same power on flat road, and the reason every answer here comes with a band: riders differ from these typical values by more than most equipment is worth.