I wonder - has there been any evidence about whether she'd ever rented a car before? I agree with you, and I think the fact that rental car mileage is so carefully recorded when you pick up and drop off the car is the one major thing she overlooked in concocting her "alibi." Her efforts seem more geared toward minimizing the TIME she spent (e.g., lying about when she left home, saying she got lost and slept in the car, etc.) rather than the DISTANCE she'd covered.
I just can't help but wonder if she honestly didn't know that the mileage would give her away... just like she didn't know that photos can be recovered from a memory card even after they're deleted... just like she didn't know that forensics would be able to determine that the knives in the dishwasher didn't match TA's wounds... just like she didn't know that gun owners typically keep "accessories" like ammunition...
High IQ or not, her "high school dropout" lack of forward-thinking skills seems to crop up quite a bit.