Rental cars in Europe can appear resonably priced and jump to unbelieveable when you return the car to the agency. Many rental car companies in Europe will not rent cars to people who are 70 or older. The first rule is to rent the car from America and not when you arrive in Europe. Renting a car in Europe can be as much as $200 to $300 a day.
First, ask in advance for the total charges for renting the car. Somthing as simple as sharing the driving with someone else can add $80 to $100 to the bill. There are always additional fees for car rental in Europe.
If you are using a credit card, make sure you know the insurance coverage the card is providing. Some cards don't cover you in specific countries, like Italy. Make sure your name is on the rental agreement and not a travel companion.
Read the fine print of the rental contract before you sign it. Make sure you know your responsiblities. Ask for an English version if you don't speak the language.
Check the car you are renting inside and out for any damages and make sure they are recorded. You will be held responsible for all damages.
Before renting extra equipment, consider buying it. For example, if you own a portable GPS, purchase and download a map for Europe instead of renting a GPS.
It is not unusal for rental cars in Europe to use diesel gas. Make sure you know the word for "diesel". Filling your tank with regular gas will damage the car and you will pay for it.
Fill the gas tank before you return the car. Since gas is very expensive in Europe, it could cost you double the amount if the agency fills the car.
Return the car when the agency is open and manned. This will insure that you'll get a written record of the car's condition. Make sure you settle all disagreements in writing before you leave.
Do not leave the agency until you have a final itemized bill that you agree with.
Remember that all traffic tickets you receive will be billed to your credit card by the agency. Traffic cameras are eveywhere in Europe, so don't be surprised if your credit card is billed 6 months after you return home.