JSFiddle - React, Tailwind, and code Playground

HTML

<label for="speed">Speed (kmh)</label>
<input type="text" id="speed">
<div>
  Speed (mph) <span id="output"></span>
</div>

JavaScript

var el = document.getElementById('speed')
// Beware that event.toElement.value returns a string
el.addEventListener(event => addConvertedSpeed(event.toElement.value));

function addConvertedSpeed(kmhSpeed) {
	mphSpeed = km2miles(kmhSpeed);
  document.getElementById()
}

function isSpeedLimit(speed, limits=[15, 40, 90]) {
	for(let limit in limits) {
  	if(speed === limit) {
    	return true;
    }
  }
  return false;
}

function km2miles(km) {
	if(typeof(km) === 'string') {
  	km = parseFloat(km);
  }
	return Math.floor(km/1.60934);
}