I had this same question in the Thread on Receipts. The reason didn't hit me until I read your question. It goes to bolster her alibi that she took the cans to save money. It was a set-up for her claim of wanting to save money. However, it kills her claim that she took the 3rd can back to Walmart.
If you look at the amount of gas she bought in SLC, Winnemucca, and Sparks (total of 42.18 gallons), she had enough gas for about half the entire 6 day trip. The 2006 Focus has a 14 gallon tank and gets 23/31, the 2007 Focus also has a 14 gallon tank and get 24/33, the 2008 Focus has a 13.5 gallon tank and gets 24/35. The trip from SLC to Sparks appears to be highway, so I'll give the upper end of what she could get milage-wise, but realize it actually was a little less. 2006 Focus: @31 mpg she could have traveled: 1326.8 miles; in the 2007 Focus she could have traveled: 1391.94; in the 2008 Focus she could have traveled: 1476.3 miles. I'm not sure what model year she rented, but those are the most likely years. Anyway, the amount of gas she bought on that stretch was way more than she needed. It wasn't a whole lot of savings, so what was the real reason. The trip from SLC to Redding could have been made with 2 fill-ups, why the extra 15 gallons of gas?