your code and check right control u find
DropDownList drpPlateNum = (DropDownList)Default._def.GRDTech.FooterRow.FindControl("drpPlateNum");
int readDepartureKM = Convert.ToInt32(txtAddFROMKMREAD.Text);
TextBox txtAddTOKMREADING = (TextBox)Default._def.GRDTech.FooterRow.FindControl("txtAddTOKMREADING");
To
<pre lang="c#">
DropDownList drpPlateNum = (DropDownList)GRDTech.FooterRow.FindControl("drpPlateNum");
int readDepartureKM = Convert.ToInt32(txtAddFROMKMREAD.Text);
TextBox txtAddTOKMREADING = (TextBox)GRDTech.FooterRow.FindControl("txtAddTOKMREADING");
</pre>