Subtotal Function Problem

diesel fuel subtotal example.xlsx
please see the attached file.
I have performed three separate subtotals, at each change in vehicle, the first subtotal is for the the quantity, easy enough.
I then added two columns next to odometer, labeled odo1 and 2.
I then subtotaled odo 1 for a min function and odo 2 for a max. My plan was to then compute miles driven in the miles column by subtracting min from max odometer readings.
I would like to automate the subtraction of odo 1 from odo 2 to get total miles driven for that truck that month.
The number of rows vary each month, at first we thought of subtracting the last line from the first but that did not work as we couldn't figure out how to handle the varying number of lines for each vehicle and came up with this method instead.
Is there another way of doing this?
The actual sheet is 5562 lines long with varying numbers of lines for each vehicle each month.