calculating mileages for several vehicles

This is a current spread sheet that I am using to track vehicle mileage (sample of information)
I have 5 trucks and their unit # is listed in the 2nd last column.
I want to subtract their previous recorded mileage from their most recent recorded mileage to get the total miles they had driven on a particular fill up.
Column 2 is the subtraction (manually now), column 6 is the truck ID #, and column 7 is the mileage recorded for that fill up.


4-Apr-14 255 60.951 1.247 $76.01 9 66388
4-Apr-14 553 90.620 1.247 $113.00 5 286014
4-Apr-14 319 80.068 1.249 $100.00 7 73776
4-Apr-14 179 44.387 1.239 $55.00 8 60450
12-Apr-14 475 113.219 1.369 $155.00 8 60925
12-Apr-14 481 116.876 1.369 $160.00 9 66869
12-Apr-14 468 117.293 1.369 $160.57 7 74244
12-Apr-14 373 60.205 1.369 $82.42 5 286387
19-Apr-14 174 41.019 1.341 $55.01 9 67043
19-Apr-14 364 104.401 1.341 $140.00 6 89158
19-Apr-14 353 80.545 1.341 $108.01 8 61278
19-Apr-14 530 89.723 1.341 $120.32 5 286917
19-Apr-14 306 75.326 1.341 $101.01 7 74550

Right now I am manually calculating the 2nd column by finding the previous entry for that truck.
The spacing changes because some trucks get used more and filled up more often.
I keep thinking there must be a way to do a lookup to find the previous truck number and then return the second column of its mileage, but I think the sortation of my columns is backwards to do that.

The spacing on the columns looks better when I am typing the post... they get tight when I actually post it.

Can anyone help out?
Thanks
Chad