There are several ways to do this - but all require a field with a common value for all records in the same period. You could use a calculation field (result is Date) along the lines of =
YourDate - DayOfWeek ( YourDate ) + 1
(this assumes your period is a week starting on Sunday)
Once you have that, you can define a sub-summary part based on the common date. Place your summary field in this part abd sort the records by the common date field.
Another option is to define a self-join relationship matching on the common date, and using another calculation = Sum ( related::mileage ) to get the weekly total.
Comment, thanks for the two solutions. However, I left a little bit of information out; I am a complete novice at this. I tried for a good hour to figure out your solutions to no avail. Is there a better way to explain it to a novice or a book that I could look at? I am sure that your solutions work, I just can't make them work.
Thanks for your help.
I don't think I can answer such a general question. You need to get familiar with the basics of Filemaker. Do the tutorial and read the help. Then, if you run into a specific problem or question (such as "I did this, but I am not getting that'), post it here.