How to get data from previous group

I have a very simple crystal report. The purpose of my report to do some type of audit on the vehicle by checking the vehicle's Odometer for a given day.

The report is grouped by 1) WorkDate, 2) Vehicle#

I displayed the information on Group#2.

Example:

Work Date Vehicle# Start Odometer EndOdometer

07/01/2013 1903 192211 192270
07/01/2013 1904 100215 100230

07/02/2013 1903 192271 192280
07/02/2013 1904 100232 100250


When I have new Work Date, I now want to calculate the variance of the Odometer for a vehicle by taking the current "Start Odometer", in this case, 192271 minus the EndOdometer from previous work date, in this case it would be 192270. So, the variance for Vehicle# 1903 would be (192271 - 192270) = 1.

My question is; how can i retrieve EndOdometer from previous day?

Thank you in advance for the help!