solo debes manejar la propiedad tabindex....
<html>
<head>
</head>
<body>
<form name="forma" method="post" action="">
<table align="center" width="100%" cellpadding="0" cellspacing="0" border="0">
<tr>
<td width="21%" bgcolor="#CCCCCC"> Nivel de aceite motor <font size="2" face="Verdana">
</font></td>
<td width="28%" bgcolor="#CCCCCC"><select name="nivacea" style="width:127px" tabindex="1">
<option value="Full"> Full</option>
<option value="Crítico"> Crítico</option>
<option value="Medio"> Medio</option>
</select></td>
<td width="46%" bgcolor="#CCCCCC"><select name="nivaceb" style="width:127px" tabindex="4">
<option value="Full"> Full</option>
<option value="Crítico"> Crítico</option>
</select></td></tr>
</table>
<table align="center" width="100%" cellpadding="0" cellspacing="0" border="0">
<tr>
<td width="21%" bgcolor="#CCCCCC"> <font size="2" face="Verdana"> Nivel de combustible
</font></td>
<td width="28%" bgcolor="#CCCCCC"><select name="nivcoma"style="width:127px" tabindex="2">
<option value="Full"> Full</option>
<option value="Crítico"> Crítico</option>
</select></td>
<td width="46%" bgcolor="#CCCCCC"><select name="nivcomb"style="width:127px" tabindex="5">
<option value="Full"> Full</option>
<option value="Crítico"> Crítico</option>
</select></td>
</tr></table>
<table align="center" width="100%" cellpadding="0" cellspacing="0" border="0">
<tr>
<td width="21%" bgcolor="#CCCCCC"> <font size="2" face="Verdana"> Luces Delanteras </font></td>
<td width="28%" bgcolor="#CCCCCC"><select name="lucdela"style="width:127px" tabindex="3">
<option value="Buen Estado"> Buen Estado</option>
<option value="Mal Estado"> Mal Estado</option>
</select></td>
<td width="46%" bgcolor="#CCCCCC"><select name="lucdelb" style="width:127px" tabindex="6">
<option value="Buen Estado"> Buen Estado</option>
<option value="Mal Estado"> Mal Estado</option>
</select></td>
</tr>
</table>
</form>
</body>
</html>
=)