VO2 max is the maximum rate at which your body can consume oxygen during intense exercise, measured in milliliters of oxygen per kilogram of body weight per minute. It's a genuine physiological ceiling -- a measure of aerobic capacity -- not a measure of speed or technique directly.
Why two runners with the same VO2 max can have different paces
VO2 max sets an upper bound on aerobic capacity, but actual race pace also depends on running economy (how efficiently you use that oxygen at a given speed), lactate threshold, and pacing strategy. Two runners with identical VO2 max scores can post meaningfully different race times because one converts that aerobic capacity into forward speed more efficiently than the other.
Estimating VO2 max without a lab
True VO2 max is measured in a lab with a metabolic cart during a maximal exercise test. Field estimates (from a timed run test, or from wearable-device algorithms based on heart rate and pace) are reasonable approximations but carry a real margin of error compared to the lab-measured number -- useful for tracking your own trend over time, less reliable as an exact absolute figure.
Converting between pace and speed
Pace (minutes per mile or kilometer) and speed (miles or kilometers per hour) describe the same thing from two different angles, and runners tend to think in pace while cyclists and general fitness contexts tend to think in speed. The speed-to-pace calculator converts directly between the two, and the running pace calculator works from a target distance and finish time to the pace you'd need to hold.