Booking three to six months before you travel is usually ideal. Prices tend to be most competitive during this period. There isn’t much benefit to booking further ahead. If anything, prices tend to be higher a year ahead. 

If you’re booking nearer to your trip, and you’re happy with one of the more common car types – e.g. something in the minieconomy or compact car groups – it’s rarely a problem to book a car closer to your trip. City Car Rental have plenty of these models, although prices can rise late in the day if demand is high.