Jet 4.0: columns data type in Excel file

Under Jet 4.0, by default, it determine the column data type by first 8 rows.

If you want to the driver determine the data type considering all rows, you can edit the "TypeGuessRows" in Registry Editor.

Go to the Registry Editor by typing "regedit" in cmd, 

Under Registry Editor, go to HKKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Jet\4.0\Engines\Excel

Then, Edit the value "TypeGuessRows" to 0 which means consider all rows to guess data type.