It represents the relationship beween the size of the front chainring and the size of the rear sprocket. For example, if you have 40 teeth in front and 20 in the back, the ratio is 2.0 or 2:1.

It represents the relationship beween the distance travelled by the pedals and the distance travelled by the bicycle.

They're equivalent to the drive tire radius, multiplied by the Gear Ratio.

The distance travelled by the bicycle for a full turn of the crank. This value is also known as rollout or roll out.

Most fixie riders don't use brakes (even if they have one as a safety backup for emergencies). One way of stopping quickly is shifting your weight forward to reduce traction on the rear wheel and stop pedaling. The rear wheel will lock and the bike will stop. Since you always brake with your feet - and the crank - in the same position and that your rear wheel is in a strict relationship with the chainring, you can predict how many spots on your rear tire will wear while skidding. These spots are called skid patches.

Regular skidders always have the same stance, strong foot in the back, when skidding. Ambidextrous skidders are able to do it with their weak foot as well.

The more the better

When you have an even number of Skid Patches, skidding in your weak stance will wear the tire at the same spots as if you were skidding in your strong stance. If you have an odd number of Skid Patches, the Skid Patches of the weak stance fall in between those of the strong stance, hence doubling the number of patches.

I'm not qualified to answer that. I chose 90 RPM because there seems to be a general consensus about the 80-100 RPM range, and it's a rhythm I'm comfortable with, as are most cyclists.

There are over 200 tire sizes in the app.

To add many requested tire sizes which had difficult to obtain measurement information, I needed to do that. All approximations are within 3% of the previously used manufacturer values. Considering that rollout varies with bicycle load, tire pressure, and from manufacturer to manufacturer for a same sized tire, ISO size approximations are better.

47x17T, 170mm cranks, 26 x 1.2 slicks, V-brake in front.

26Tx34T. And I sometimes wish I had a smaller chainring.