console
var rate=new Array(1.609344,0.9144,3.7854118,0.4535924);
function cmpl(t,i,o)
{
o.value=rate[parseInt(t)]*parseFloat(i);
}
<body>
<div class="top" style="height:70px;border-bottom:1px solid #ddd;">
<img class="logoIndex" height="60" src="https://www.zenitour.com/img/logoIndex.png" style="margin-left:10px;margin-top:5px;">
</div>
<div class="content" style="margin-left:25%;margin-top:20%;height: 78%;">
<form>
输入:<input type=text name=i value="" onchange="cmpl(this.form.t.value,this.form.i.value,this.form.o);">
<select name=t onchange="cmpl(this.form.t.value,this.form.i.value,this.form.o);">
<option value=0>英里到公里
<option value=1>码到米
<option value=2>加仑到升
<option value=3>磅到千克
</select><br>
输出:<input type=text name=o value='' readonly><br>
</form>
</div>
<div id="bottom" style="text-align:center;border-top:1px solid #ddd;">
<span class="centerspan" style="font-size:10px">Copyright © 2017 Zenitour lnc All Rights Reserved 版权所有-哲途教育 渝ICP备13002955号-2</span>
</body>
html,body {
width: 100%;
height: 100%;
margin: 0;
padding: 0;
}