Hi,

You can also use an expression box to subtract. Try converting the total
miles field to an expression box, then enter the following formula: StartOd -
EndOd
where StartOd and EndOd are the fields in your table. Also make sure that
StartOd and EndOd are of a numeric type (whole number or decimal) and not
text. You can change this through the properties dialog box of the
corresponding fields in your form's data source.

I don't believe you can do a sumif without using custom code. Have a look at
the Expense Report sample form that comes with InfoPath. It contains code to
do additions in a repeating table.

Hope this helps.

Regards,
S.Y.M. Wong-A-Ton