There are two things Speed can refer to when it comes to indoor training.
Simply, how fast your wheel is spinning.
This value matters as a part of the power calculation. There is not a direct correlation between how fast your wheel spins on the trainer and how fast you'll go on outdoors at the same effort.
Keep in mind that if your speed reading is wrong, your power reading must also be wrong. If your power readings seem reasonable, read the next section to understand why a "weird" speed reading on its own isn't necessarily a concern.
If you use a simulation such as Zwift, the app will calculate virtual speed and virtual distance, using data including your power output, your weight, and the grade of the virtual terrain. It should approximately match what you'd achieve outdoors for a similar course and effort. This is the value you'll use to measure your progress on the bike if you want to know "How far?" and "How fast?"
ERG mode doesn't allow for a virtual speed to be calculated because there's no virtual terrain. Instead, you'll focus on power.
Why is wheel speed different on the trainer vs on the bike?
Power is is king when it comes to interval workouts in ERG mode. In this mode, you could spin your wheel at 70km/h or 15km/h, but it won't change the difficulty of the workout. Whatever your wheel speed and gear, the trainer will adjust the resistance to make sure you hit your target power level in Watts.
Reminder: Spinning your wheel faster in ERG mode improves the road feel and increases the maximum possible resistance. Shift to your top gear!
In a simulation, you use your gears as you would outdoors, while the trainer creates the feeling of hills by increasing and decreasing your resistance.
This means your wheel speed more closely approximates your wheel speed outdoors, but there are still situations where they diverge significantly. For example, if you stopped pedalling on a downhill outdoors your wheel would continue to spin rapidly, but indoors, your wheel will coast down.
- How do I get an accurate speed reading?
- My 'mileage' on the trainer is different from what I'd expect riding outdoors for the same time/effort
- I'd like to try interval ERG workouts, but I'm accustomed to measuring my rides in speed and distance. How can I find out how "far" I've come?