For POVs (privately operated vehicles), the federal government typically reimburses based on mileage between the two locations, at a rate of X cents per mile. The government typically determines the mileage based on the address at which you started and finished; thus, the route you happen to choose has no bearing on the mileage allowed. You can probably find the current rate of reimbursement on some website.
The per mile reimbursement is usually more than sufficient to cover gas costs. It also covers (to some extent) wear and tear on the vehicle. If you need to travel more than one day (based on the mileage the government expects you to cover in one day, which used to be somewhere around 350 miles/day), you will get a "per diem" to cover your expenses. It's not a lot but will allow you to stay in an acceptable (not luxurious) hotel and eat decent (not gourmet) meals.
Or that's how they used to do it. If it's changed, someone please correct me!
