I have purchased an Elite Direto Smart Trainer.
When i connect it to Edge 820 as Indoor Trainer Sensor, and the wheel circunference set to 175 as Elite says, the speed is only of 2-3 km/h, very low.
If I disable Indoor Trainer Sensor and enable Cad/Speed sensor of the same Elite Direto trainer with 175 wheel circunference, the speed is correct.
What is the correct wheel circunference configuration for good speed readings.
This trainer, like many others, estimates cadence by looking at power pulses as you pedal, two power pulses (usually due to the left and right pedal) is interpreted as one crank revolution. This isn't very reliable and depends on how you are pedaling. A separate cadence sensor that directly senses crank rotations will be much better, particularly with rapid cadence changes and pedaling at high cadence with low resistance. If you are having problems with the cadence being reported under normal pedaling against resistance, it'd probably be best to contact Elite.
for what it is worth I have the same setup (820 and Direto) and do not have either a speed or cadence issue. The one thing I did do was download the Elite app for my phone, and setup the device from there. I don't know if that matters, but I get cadence that seems about right, and speed that is in the ballpark. (But speed seems to still be a bit low. I think it bases itself on cadence and does not take power into account.
It depends on what device you are using to capture the speed/cadence data.
If you are using the MyETraining app, use your normal well circumference e.g. 2086 for a 700 x 20C
If you are using a Garmin device, use a compensated value. For the Direto, divide the wheel circumference by 12.1 and use this new number as your wheel circumference in your Garmin device setting.
This page lists the various conversion factors