Crystal Reports How Do I subtotal the change in a numeric field?

Message

Ok, well, I didn't see an answer to this after searching for 30 minutes, so here goes:

I have the following table:

Unit ID    Department    Repair Date     Repair Odometer
101         01            2009-01-01     100,000
101         01            2009-03-01     104,000
101         01            2009-05-29     108,400
102         01            2009-01-13       92,000
102         01            2009-03-14       96,500
102         01            2009-06-01     101,000
103         02            2009-01-01      45,000
103         02             2009-06-01      59,000

I need to be able to summarize how many miles each vehicle traveled, then summarize the subtotal. 

So far, I have been able to create a formula that gets the mileage at the start date and the mileage at the end date and subtracts the two. This helps avoid any data entry mistakes made in between the two dates (which is quite common with our user base, so I'm trying to make this foolproof).

numbervar MinMiles;
numbervar MaxMiles;
select {Repair Date} 
case Minimum ({Repair Date}, {Unit ID}) :  MinMiles := {Repair Odometer}
case Maximum ({Repair Date}, {Unit ID}) :  MaxMiles := {Repair Odometer}
numbervar Miles := MaxMiles - MinMiles;

This works great when I put it in the group footer for the Unit, but I can't get a subtotal in the next level up, the group for the department.

Is there anything that will total the change in a field, and then is there any way to subtotal the sum of all the changes? I've spent the last day and a half trying 20 different ways to get this to work. So far they all have the same results, and this is the simplest formula I'd been using. Everything else was having to use a lot of if Unit ID <> Next (Unit ID) logic. 

Edited by sundevilkid - 08 Dec 2009 at 2:31pm