# What is the difference between a speedometer and a power meter in terms of performance measurement? [duplicate]

If I cycle the same route every day, what would be the benefit of having a power meter over a speedometer?

Can't I just measure my speed and compare it against my last run and see if I've done better?

## marked as duplicate by mattnz, paparazzo, Criggie♦, David Richerby, Neil FeinOct 9 '17 at 14:29

• You say tomatoes I say potatoes @Paparazzi – teslajin Oct 5 '17 at 16:06
• Utimately, both together will give you more information than either on its own. Imagine headwind+more power in = slower overall speed compared to the return where you have tailwind +normal power in = higher speed. Which is the better "performance" the faster speed or the more work done ? – Criggie Oct 5 '17 at 21:47

Can't I just measure my speed and compare it against my last run and see if I've done better?

Yes, but it's only directly comparable if the wind (and surface, and traffic ... but particularly wind) conditions are identical. Since you don't usually have an accurate measure of the wind vector relative to your movement at each point on your ride, you're going to have a pretty rough comparison at best.

Even that rough comparison is available only if you take exactly the same route, whereas a power meter can give you some idea how you're doing even on a different route with a different elevation profile.

Also, just measuring speed doesn't tell you anything about the power distribution through your pedal stroke, or the (im)balance in work done by your two legs.

Whether all that extra information is actually useful or interesting to you, or worth the cost of a power meter, is naturally a question only you can answer.

Time would only be an equivalent measuring stick if all other factors were kept constant. However, things like the wind direction and speed, any extra load you may have (like a backpack), etc all affect how much power you need to apply to get that same time. Even other bike traffic or random congestion on your route will impact your time, either by causing more braking or just limiting your top speed.

All keeping track of the route time does is tell you how long it took. A power meter will measure how much work (from a physics perspective) you did during that time. And higher performance is related to being able to do more work.

• Don't forget traffic. Even on a bike path the other users of the same space have quite an effect on your timing. For example my regular commute is much slower on bin day. – Chris H Oct 5 '17 at 18:32
• Good catch, updated. – Ross Oct 5 '17 at 19:58

It is often helpful to be able to distinguish which elements of your performance were a result of an improvement in your physiological capability (i.e. your fitness as measured by your power output), your tactical abilities (e.g. pacing) versus other factors (e.g. weather, bike set up etc).

There are two main differences in the utility of speed and power as a means to measure these changes in your abilities:

• The first involves the impact of many uncontrollable variables which affect your speed but not your power output, most notably differences in aerodynamic drag factors such as wind conditions, air density and your coefficient of drag.
• The second is the significant difference in the resolution of speed and power in detecting differences in your physiological capabilities (i.e. fitness).

Even the most subtle changes in wind conditions, air density, and even your position on the bike and clothing can result in quite a different power requirement for the same speed.

For example, a non-noticeable headwind of just 1m/s (that's so slight you could just feel it) would mean the power required to ride along a flat road at the same speed, say at 30km/h, when compared to no wind at all, increases by about 20%!

As you can see from the chart above, even a minor difference in wind has quite an impact.

Unlike speed, your power output is entirely down to you alone, being a result of your physiological capability and how hard you choose to ride relative to your ability.

The second factor I mentioned is the difference in the resolution of speed and power measurement to detect changes in physiological capability. This is due to the cubic (rather than linear) relationship between speed and power. This is shown by the curvilinear plot shown in the chart for flat road riding.

In other words, changes in speed (on flatter terrain) require much larger changes in power, in particular at speeds above 25km/h where the plot steepens significantly.

The result of this is that differences in physiological capability (i.e. fitness) will be reflected in power measurement but the speed differences may be too small or subtle to be able to reliably detect a change in physiological capability, and will of course also be affected by the noise of uncontrollable variables.

So while speed is OK to measure large differences in ability on average over a period covering many many rides on the same route, it does not have the resolution or reliability to detect short term and/or more subtle changes in fitness.

As such power is superior as a measure of both your physiological capability and relative effort level, particularly when you reach the stage when changes in fitness are not large, which is typically the case once you've been riding and/or training consistently for a year or two.

Whether having such increased resolution and reliability in measuring your capability matters to you is however somewhat subjective and individually variable.

Speed may be considered to be the ultimate output of the cycling process and therefore performance would be measured with a chronometer/speedometer. And that would be it.

Watts may be considered as an input in the cycling process. A power meter will provide an intermediate measure of effort, which is but one of the speed determinants.

Several factors may be considered, some belonging to the environment (wind, traffic, etc.) some to the rider (aero position, state of mind, proper pacing, etc.), and some to the bicycle itself (aero, weight, inertia), such that speed will vary greatly for a given power level.

You may also want to measure other "performance" metrics, such as how efficiently you can convert food to Watts. Heart rate monitors are well known devices. Muscle monitors are appearing over the horizon. They could conceivably be measures of metabolic performance.