I don't know a lot about the SLC fill-up, but is it possible that she did this because she was considering revisiting the crime scene to make sure that her clean-up was adequate? Two days after the murder, and no one had found Travis' body yet: maybe she was having excruciating "what-ifs" and came very close to driving back to Mesa to double check her clean-up, to make sure that she hadn't left anything behind.
Somebody said this timeline isn't right, but this is what I have to go on. I added the miles and driving times to this scenario. Can somebody tell me where the extra stop for gas was made, an how much gas was purchased? I'm trying to figure out where the lost hours of her trip come into play.
June 6, 2008:
Salt Lake City, Utah (to Winnemucca NV is 353.8 miles a 5 hours and 7 min drive)
Arias makes two gas purchases at Tesoro between 3:57 a.m. and 4:05 a.m. The first one was for $41.18. The second one was 8 minutes later for an additional $36.98.
Winnemucca, Nevada (to Sparks NV is 162.9 miles and 2 hours and 19 mins drive)
Arias spends $49.91 on gas at a Flying J Travel Plaza. She also spends $4.90 on other items. There is no time stamp on the receipts for these purchases.
Sparks, Nevada (to Redding CA is 198.4 miles and 3 hours and 28 mins drive)
At 10:34 a.m., Arias buys gas at a 7-Eleven. It is not clear how much gas she bought. She also makes two purchases at an In-N-Out Burger at 10:38 and 10:43 a.m.
June 7, 2008:
Redding, California
At 12:20 p.m., Arias buys gas at a Valero.
At 1:08 p.m., Arias returns her rental car. She originally planned to return the car on Friday, June 6 at 8 a.m. Some time during the week, she apparently extended her rental until 9:30 a.m. Saturday, June 7. The rental agent testified that the car appeared to have "Kool-Aid" stains on the passenger seat and back seat and all the floor mats were gone.

In all, Arias traveled 2,834 miles.