A useful car-hire comparison starts with the trip itself: where you will collect the car, which countries you will visit and where you will return it. The same company can offer different conditions at different locations.
Read the insurance terms and find out what you would pay if the vehicle were damaged. The European Union's consumer guidance also advises telling the rental company in advance if you plan to cross borders, so it can confirm permission and the right documents and cover.
Check the fuel policy, mileage allowance, deposit, extra-driver charge and any one-way fee. Save the booking terms and photograph the car at collection and return.
Compare the final price for the same dates and conditions. A cheap headline rate is only useful when the agreement fits the journey.