Booking a flight and rental car together offers the convenience of having your travel reservations stored on one website or with one company. Although you will likely receive separate confirmations from the airline and rental car company, you will be able to access your entire reservation in one place. Also, combining your bookings often means that travel companies can pass on preferential rates from third-party vendors and save you a substantial amount of money. If your travel preferences are flexible, check with several different airlines to get the best car-and-flight deal.
If you book your flight and rental car well in advance, you'll have your pick of seats and cars in addition to access to low rates. You'll also enjoy the peace of mind that comes with knowing your reservations are complete. The closer your travel dates -- the later you wait to book -- the more reservations will have already been made with both your airline and car company of choice. Therefore, prices are likely to be higher for the remaining inventory.