My Filzer dZ2L Cycling Computer was bought at Mountain Equipment Co-op for $17.50 yesterday.
The computer measures your speed, trip distance, total distance, how fast you are relative to your average speed, and whether you’re accelerating or decelerating.
The computer is wired which means you need only one battery. I’ve used a wireless one before. I stopped using it because its receiver’s battery runs out even though I used it lightly.
I bought it because it’s reliable based on reviews and displays your speed in km/h. Based on its reviews, it’s more reliable than some of the similarly priced models. It’s easier to pace yourself with a speedometer than by feel which is beneficial when you want recovery days.
You can use the speedometer to test if your speed is affected by different tires, inner tubes, inflation pressures, road conditions, and postures and see if they affect your speeds. I use the 700x38c Schwalbe Marathon Greenguard tires which are considered fast rolling.
I still use Strava on my tablet to record the other details such as time, elevation, maximum speed, and average speed.