Present your auto rental voucher. Multiple bookings with the same supplier (perhaps you made a reservation online which was never cancelled) are confusing to rental car suppliers. If a written booking confirmation is not presented, the rental agent may locate the wrong reservation and a different — higher — rate could be charged.

Call the toll-free-from-Europe help line. Provided you book with Gemut.com, most issues that arise at the rental counter can be solved in minutes via a toll-free-from-Europe, 24/7 customer service help line (the number is under Terms & Conditions in your voucher). Let's say you booked an automatic transmission car but one is not available when you arrive. Call the number. Promised an upgrade, but the rental agent offers a Lupo? Call the number. It's your safety net.

Decline insurance. Most credit cards issued by North American banks offer CDW/Theft protection for auto rental in most of Europe. There are exceptions like Italy and Ireland, where the customer must purchase full-coverage. In all other countries, I recommend clients decline CDW/Theft in favor of the coverage offered by the credit card. Call your credit card issuer for details. Be sure the contract you ultimately sign does not include this optional insurance. If the counter agent says you have to purchase insurance, call the toll-free number mentioned above.

Decline pre-paid fuel. This is a bad deal. The offer works this way: you pay for the first tank and return the car empty. Not as easy as it sounds and who wants to be driving around on fumes, especially when headed for a European airport to catch a flight home. Any fuel left in the tank at the end of the rental is yours, but you won't get a refund.

Ask for instruction on vehicle operation. It once took me a full five minutes just to figure out how the windshield wipers on a BMW. Radios, too, are often not user-friendly and may have unfamiliar features. If you get a car with a GPS be sure it's set on English language mode. Finally, be sure you know whether you have a gas or diesel engine. Figure on a charge of about $400—and major inconvenience— if you fill the tank with the wrong fuel.

Inspect the car. If the car is dirty inside or out, refuse it. Check for obvious and not-so-obvious damage (even small scratches). Make sure any damage is noted in writing. If you have a digital camera, it's a good idea to take pictures of the car from all sides and again when you return the car. Visually inspect tires for wear and inflation level. When in doubt, call the toll-free help line.

Get quote on a rental car in Europe

Hear this: you want a rental car contract that does not include collision damage waiver (CDW)–LDW (loss damage waiver) in Europe—or theft insurance. Naturally you want the coverage, you just don't want it from your rental company. Why? Because, as the UK magazine Which? Holiday says:

Most car-hire fees automatically include some kind of car insurance, however, in most cases, these policies protect the company and not the consumer. Customers with damaged cars often have to pay out large excess fees. The article goes on to declare, The excess (deductible) fee that the consumer would have to pay in the event of an accident if they had rented a car from any one of the five most popular car-hire firms in the UK and collected their car from an airport in Ireland, ranged between €779 ($1207) to €1,947 ($3,018).

We've been saying this for years, but it's nice to hear it from another source. When renting a car through a Europe-based company, such as Argus or Nova, or directly from a supplier such as Europcar, the insurance is almost always included, but with a high deductible (excess). In addition, this included insurance, which "protects" the renter from damage and theft, often excludes damage to many parts of the car including the windshield, wheels, roof, undercarriage, tires and interior. When you make a booking online from all the Europe-based companies I've seen, this information is unearthed only after a diligent search and the clicking of several links. And, just as soon as you sign a rental car contract including theft and CDW, it is extremely likely that your credit card insurance will be invalidated, because, except in rare cases, in order for your free, zero-deductible credit card insurance to be in effect, you can't have any other coverage...regardless of the amount of that coverage's deductible.

Most gold, platinum, Amex, and Diners' Club credit cards offer free CDW/theft. Visa's Website contains a fairly clear and straightforward description of what its insurance does and does not cover. Most ordinary passenger vehicles are covered but travelers who plan to drive a nine-passenger van, a Porsche sports car, or a Mercedes-Benz, BMW, or Audi, with a sticker price over about $45,000, should check with their credit card company.

Get quote on a rental car in Europe.

The price of a liter of gasoline today in Berlin is about €1.462. I'll save you from the math: that translates to about $7.47 per gallon. You can always find up-to-the-hour prices for gasoline and diesel fuel at Gasoline-Germany.com.

You can expect to get about 35 miles per gallon in a compact rental car in Germany. Thus if you average 100 miles per day on a 14-day vacation you can expect to use 40 gallons of gas at a total cost of about $299.

Get quote on a rental car in Europe.

Sam paid for his auto rental in advance before leaving for Europe. He got an excellent dollar-guaranteed rate and avoided the three-percent foreign transaction charge that his credit card would have imposed had he waited to pay in Europe. When he finished the rental, however, the rental company handed him a bill for the local road/registration tax, a fee of about 10 euros. Since Sam was returning to the U.S. and wanted to get rid of his euros, he paid in cash. Big mistake. You see, there was also minor damage to the car, which the rental car company routinely billed to his credit card a few days later. When the rental company sent Sam the bill in the U.S., he turned to his credit card for reimbursement under their free CDW coverage. In order to be eligible for credit card insurance, however, the renter must pay the entire cost of the rental with the credit card. Since Sam had paid for an item (the road tax) on the car rental invoice in cash, there could be no reimbursement. He had to pay for the damage himself. Fortunately, the car was only scratched and the repair bill was about $630. Had it been totaled or stolen, Sam would have been on the hook for its full value.