You will generally get the best rental car rates online as opposed to renting at the kiosk
or in person.... mostly because when you show up at the counter you have less availability of vehicles.
It's not necessarily that online has cheaper rates, as a rule. it's just that by booking online you are booking in advance of your trip. The more in advance of your trip you rent, the more cars there are usually available... that's when you usually will get the best car rental rates.
Although there are such things as specials for last minute car rentals, it doesn't happen very often. Because the no show rate (people who reserve but don't pick up their cars) for the car rental industry is around 30%, the rental companies may even overbook to make sure that they can get as many vehicles rented a day as possible.
Generally, you will get the best rental car rates by booking at least 2 weeks or so in advance... sometimes even better if you book 4-6 weeks in advance. When you book several weeks in advance, you can also keep checking for Last Minute Car Rental Specials, and rebook your reservation to take advantage of a lower rate. A rebooking tip is to try and change your return time slightly (still keeping the same number of rental days), this should kick in the new rate.... if not, just call the rental company's toll free number to get assistance from a reservation rep. Bottom line... to get the best car rental rates,try to book in advance; When you show up at a counter with no reservation, you run the risk of there being no cars available at all or only a few left, and the prices on those few that are left will be high.
Pages from this site, relating to this topic:
Tips on finding Car Rental Specials
Making Car Rental Reservations