Top 12 car rental tips

The most important things to know before booking a rental car

1. Book from home. Don't wait until you're over there to rent a vehicle. It is invariably cheaper to rent a car from the United States. Most major European rental agencies are now part of, or affiliated with, the big U.S. agencies (Hertz, Avis, etc.), so going direct to the European ones doesn't yield a better deal. » more
2. Use an aggregator to determine a base fare. Research the going retail rates at various major rental outfits, booking sites, discounters, and travel agencies by using a meta–search engine called an aggregator. Then see if you can beat them with a consolidator (next step). » more 
3. See if a consolidator can beat those prices. Auto Europe (www.autoeurope.com) offers consistently lower prices than the major agencies—even though you'll actually be getting an Avis or Europcar or whatever. This is almost always my first choice when I need to rent in Europe, and since they now do leases as well, it's the best one-stop-price-shopping for the best option. » more
4. Always get the full rate. Rental companies love to stick it to you with low initial per-day rates, then add on all sorts of bells-and-whistles at the last moment (insurances, taxes, road fees, one-way charges to pick up in one city and drop of in another, etc.).
5. Do not fall for the "fuel plan." in which they pre-charge you for a full tank of petrol. Yes, this saves you the "hassle" of having to gas up just before returning the vehicle, but it also means that, unless you coast in on fumes, you are gifting them whatever is left in the tank. In other words, you always end up buying more fuel than you use. 
6. Inspect the car before you drive off. If any pre-existing nicks, scratches, dents, or other damage is not indicated and initialed by a local employee on your rental form before you leave, you will be liable for it when you return the vehicle (oh, and they will charge you an arm and a leg, believe me; even with CDW, you will still have a deductible to satisfy before the insurance kicks in). » more
7. Don't rent more than you need. We're talking both the time period in which you'll need the car, and the kind of car you'll need.
   • First, rent for as short a period as possible. Don't rent a car for the full two weeks if you're spending your first four days in London. You don't need a car in London. In fact, you don't need (or want) a car in any major city, where public transport is fast, efficient, and cheap (see Tip 6 below for more). Arrange to connect major cities by train, and just rent the car for the shorter period when it is truly useful (the Cotswolds, the Lake District, the Salisbury Plain, the Highlands of Scotland—pretty much, anywhere countryside). » more
   • Second, don't rent more than you need when it comes to the car itself. A smaller car will give you better gas mileage, cutting down costs (and make it easier to navigate the winding road and narrow streets). If you can drive a manual, stick-shift is always cheaper than automatic (and also gives better gas mileage). » more
8. Forget driving in cities. See above. Not only does London have its congestion charge, which makes driving into the city center expensive, it just makes no sense to have a car in a city—traffic is crazy, many streets are pedestrianized or one-way, and parking is impossible and expensive—all while public transit is wonderfully efficient. Avoiding having a car in the city also also niftily helps you avoid the airport pick-up surcharges, since the ideal itinerary has you picking up a car at a downtown office on your last day in one city (rather than at the airport when you arrive, only to drive the thing downtown and garage it for several days), and then dropping it off as soon as you arrive at the last big city. Congratulations. You just shaved several days (and several hundred dollars in rental and parking fees) off your rental period. » more
9. Look into short-term leases. If you're renting a car for three weeks or longer, look into a short-term lease. All things being equal, this will often cost less than a similar rental, plus it comes with all insurances, no deductible, and a brand new car. » more 
10. DRIVE ON THE LEFT! The most obvious rule, but the easiest to forget each morning when you get back behind the wheel. » more
11. Follow all driving rules and regulations and road signs. Drive defensively and cautiously. Yes, European drivers can seem aggressive. Do not attempt to imitate them. See, they know the rules of the road—both the meanings of all the road signs and official regulations, and the unwritten rules of how people drive in Britain (like how you should slow down and drive a bit on the shoulder to let a larger, faster car pass you, even if he insists on doing this on a blind curve of a meandering, two-lane country road). You do not know these rules, and that is a formula for trouble. Also: obey all no-parking signs. British cops have gotten brutal about ticketing (and even towing) illegally parked cars (and any unpaid tickets will find their way to you via the car rental agency, which will attach a whopper of a fee for their troubles, along with the probable late penalties on the ticket itself). » more 
12. Leave the driving to someone else. A rental car is great—but not always the right choice, even when you need to take a road trip in England. The truth is, if you plan to take a classic daytrip or excursion you can often hire a private driver and guide more cheaply than it would cost you in rental fees, gas, tolls, and sight admissions to do it on your own. Plus, this way someone else—who already knows where he's going—gets to worry about the logistics, traffic, tolls, parking, etc. You just have fun and see the sights. Also, this is the only truly fair way to tour Scotch distilleries, since otherwise the driver in your party would have to abstain. » more