I contacted the Consulate for Portugal in San Francisco. Following is their answer requiring necessity of IDP for rental cars in Portugal.
For what it's worth, I also saw that AutoEurope says IDP is required. And, AAA's information says IDP is required for Portugal. But, Portugal's representative says it is NOT required. Don't know what to tell you further except there's a lot of conflicting information out there, from supposedly professional and/or authoritative sources.
I happened to get an IDP from AAA before receiving the Consulate's answer. The IDP cost $20, plus another $US20 for photos; bring your own passport-sized photos and save the extra money, if CAA is same as AAA.
Consulado Geral de Portugal em São Francisco [email protected] Apr 20 at 6:47 PM
To albertkhenning
Message body
Dear Albert,
There's no need for an international permit to drive in Portugal.
The US drivers license is good up to 3 months.
Sincerely,
Melhores cumprimentos,
CONSULATE GENERAL OF PORTUGAL
3298 Washington Street - San Francisco - CA 94115
Website: www.PortugalinSF.com / www.en.PortugalinSF.com
Sent: Tuesday, April 17, 2018 11:18 AM
To: Consulado Geral de Portugal em São Francisco
Subject: International Driver’s Permit?
Hello,
Is an International Driver’s Permit required for driving in Portugal?
Thank you,