If you are flying to the USA from the UK and want to hire a car in the USA, it is usually cheaper to purchase the car rental in the UK in advance. It is usually but not always cheaper to obtain the car rental as part of a fly/drive package.
Be sure to obtain unlimited mileage car rental, since distances in the USA and Canada are immense.
If you plan to fly out from one airport and back from a different airport, check for any drop-off charges for the car, which can be very high. Sometimes you can drop the car at another location within California or Florida without incurring a drop-off charge.
Also, check whether driving is restricted to certain states and whether the car can be driven into Canada if relevant.
It is usually cheaper to purchase any car upgrades (to a larger size than that supplied by default with a package) when you book, rather than directly with the car rental company. As well as the size of the car, you might also consider upgrading from a two-door car to a four-door car.
Car rental is usually much cheaper for cars hired from within the USA rather than within Canada. Rental from within Florida and California is usually much cheaper than from within the other states. It is very difficult to obtain unlimited mileage in Canada.
The car rental voucher usually only covers the basic rental cost. You must pay all the other costs locally. Collision Damage Waiver (CDW), sometimes called Loss Damage Waiver (LDW), is optional but highly recommended since it protects you from being potentially liable to the car rental company for anything up to the full value of the car, even if something happens to it which is not your fault. The cost of CDW/LDW varies from state to state, but is typically $9-$15 per day, with the cheapest rates applying in California. It is sometimes possible to obtain car rental in the UK that includes CDW/LDW, in which case make sure that the rental agreement states this clearly.
You must also pay a fee directly to the car rental company for any additional drivers (typically $1 per person per day) and for the rental of child seats if needed. You must also pay the state sales tax on the entire cost, including that which the voucher covers. There are also restrictions and costs involved with young drivers.
If you have "Top-Up" insurance and normal holiday insurance, then the only additional insurance or insurance-type facility that you need to purchase from the car rental company is CDW/LDW, but they will try to sell you the others.
You will need to use a credit card to guarantee payment to the car rental company.
It is highly recommended that you take out "Top-Up" insurance in your departing country in advance. This covers your liability should you injure someone or cause property damage while driving, and it also provides you with compensation should you be involved in an incident with a non-insured or under-insured motorist. Check what level of third-party cover is provided inherently by the car rental company, since this affects the level of top-up insurance required.
Rental cars are full of petrol when you pick them up. The cheapest option is always to fill up yourself at a nearby gas station and return the car with a full tank.
When hiring a car from Dollar, check whether you can take it into Canada. There shouldn't be any problems in this respect with Avis, Hertz, or Alamo, but it is always wise to check.
Almost all operators impose a restriction against their cars being driven in Mexico. Many operators now impose a restriction against driving on unpaved roads. Some operators impose a restriction against driving their cars in parts of New York City, especially the island of Manhattan.
A British driving license is sufficient to drive in the USA and Canada; you don't need an International Driving Permit.
All normal rental cars in the USA and Canada take unleaded petrol, which is universally available.
Most, if not all, rental cars have air conditioning and a radio, and often a radio/cassette player. Cruise control, central locking, and electric windows are common. Sunroofs are very rare. All normal rental cars have automatic transmission.
Since the rule of the road is to drive on the right, the driver sits on the left. However, the accelerator is still on the right and the foot brake in the middle.
Before you return a rented car, be sure to sort out all the items that have doubtless collected during your trip. Organize your luggage in advance to make the return process smoother.