<form class="container" id="frmcarga" method="POST">
  <div class="form-row">
 <div class="form-group col-md-3">
      <label for="">Vehículo</label>
       <select  name="vehiculo" id="vehiculo" class="form-control">
        <option value="">Seleccione...</option>
        <?php
            $query="SELECT  * FROM vehiculo";
            $result=mysqli_query($con, $query) or die (mysqli_error());
            while ($row=mysqli_fetch_array($result)){
            echo '<option value="'.$row['vehiculo'].'">'.$row['vehiculo'].'</option>';
          }
        ?>
      </select>
     </div>

        <div class="form-group col-md-3">
      <label for="">Odómetro</label>
      <div class="input-group mb-3">
      <div class="input-group-prepend">
      <span class="input-group-text" id="basic-addon1">KM</span>
    </div>
      <input type="text" class="form-control numerico" name="kilometro" id="kilometro">
    </div>
     </div>
 </div>
</form>

<div class="form-group col-md-3">
      <label for="">Kilometraje Total</label>
   <?php
   $query2="SELECT SUM(odometro) AS totalkilometros FROM carga_combustible WHERE vehiculo ='spark'";
   $result=mysqli_query($con, $query2) or die (mysqli_error());
   while ($row=mysqli_fetch_array($result)){
    $row[0];

     ?>
     <input type="text" class="form-control" name="totalkilometros" id="totalkilometros"
      value="<?php echo$row[0]?>">
     </div>
<?php
 }
mysqli_query($con,$query2) or die (mysqli_error());
mysqli_close($con);
?>

$("#totalkilometros").change(function(){
  var input = document.getElementById("totalkilometros");
  var valor = input.value;
    if( valor >= 1)
    alert("Este vehículo necesita mantenimiento");
    });