You can access the terms of conditions of all the bigger players on their websites. I just did for AVIS and selected Germany as the country of origin:
Entries into Czech Republic (CZ), Croatia (HR), Hungary (H), Poland (PL), Slovakia (SK) and Slovenia (SLO) are only allowed for vehicles up to medium class and not allowed for vehicles of the makes Audi, BMW, Jaguar, Land Rover, Mercedes Benz and Volkswagen.
Entries to Italy (I), Southern France (F) and San Marino (RSM) are only allowed for vehicles up to executive class.
Porsche vehicles may be driven in Germany (D), Austria (A0 and Switzerland (CH) only.
You are not allowed to use the vehicle in Albania (AL), Azerbaijan (AZ), Bulgaria (BG), Bosnia and Herzegovina (HIB), Belarus (BY), Cyprus (CY), Estonia (EST), Georgia (GE), Greece (GR), Iceland (IS), Kazakhstan (KZ), Latvia (LV), Lithuania (LT), Malta (M), Macedonia (MK), Moldovia (MD), Montenegro (MNE), Romania (RO), Russia (RUS), Serbia (SRB), Turkey (TR), Ukraine (UA) and any country outside continental Europe.
Can I buy a product to allow me to drive in other countries?
No, regrettably, we do not offer any products that allow you to drive in other countries.
I assume similar rules are in place with the other bigger rental companies