How to add new row in gridview to find subtotal groupby vehicle no?

See more:

C#

ASP.NET

DATE O.NO VEHICLE NO PARTICULARS QTY RATE AMOUNT(Rs)
04/07/2014 9 gj656 DIESEL 8 44 352
04/07/2014 7 gj656 DIESEL 3 44 132
04/07/2014 8 hj565 PETROL 2 60 120

This is my Report here i want subtotal vehicleno wise.
So how i add a new row in gridview to find subtotal vehicle no wise????


query = "select transdate,orderno,vehicleno,oiltype,qty,price,amount from [transaction] where custid=" + int.Parse(lblid.Text.Trim()) + "and orderno in (" + slipall.ToString() + ") and invoicestatus='true' order by vehicleno,transdate";
cmd = new SqlCommand(query, con);
// SqlCommand cmd = new SqlCommand("select * from [transaction]", con);
SqlDataReader reader1 = cmd.ExecuteReader();


DataRow nr = dt.NewRow();
if (reader1.Read())
{

DataRow dr = dt.NewRow();
nextno = reader1["vehicleno"].ToString();
afterno = reader1["vehicleno"].ToString();
amt = reader1["amount"].ToString();

dr["DATE"]= reader1["transdate"].ToString();
dr["O.NO"] = reader1["orderno"].ToString();
dr["VEHICLE NO"] = reader1["vehicleno"].ToString();
dr["PARTICULARS"]= reader1["oiltype"].ToString();
dr["QTY"]= reader1["qty"].ToString();
dr["RATE"] = reader1["price"].ToString();
dr["AMOUNT(Rs)"] = reader1["amount"].ToString();
dt.DefaultView.Sort = "VEHICLE NO";


dt.Rows.Add(dr);
if (preVno == afterno)
{
IsSubTotalRowNeedToAdd = true;

}
else
{
IsSubTotalRowNeedToAdd = false;
}
preVno = afterno;
}





if (IsSubTotalRowNeedToAdd == true)
{
dt.Rows.Add(nr);
prevtot = prevtot + Double.Parse(amt);
nr[0] = prevtot;
}
else
{

nr[0] = 0;
prevtot = 0;
}
//}


GridView1.DataSource = reader1;
GridView1.DataBind();
reader1.Close();

This is how i am retrieving data from database in this code a new row is added on the top position i want new row below the vehicles of same no so please help me to get this....