Well it may be or it may not! Do you have a basis for this comment that suggests one way or the other?
Following your request, I thought I would investigate this further.
Visa & Mastercard rates were easy to find, but the AMEX rates remain hidden. There may be good reason for this. Please read on.
I chose the transaction date of Friday 14th July 2017 with a 0% bank fee (as the Citi Plus Mastercard & Citi Plus Visa Card for example do not charge a foreign exchange transaction fee either).
I selected my card in AUD$
My transaction was In US$
The daily VISA Card Foreign Exchange Rate is found here:
https://www.visa.com.au/travel-with-visa/exchange-rate-calculator.html
Using the above, the Visa foreign exchange rate was:
1 United States Dollar = 1.320174 Australian Dollar
The Mastercard foreign exchange rate is found here:
https://www.mastercard.us/en-us/consumers/get-support/convert-currency.html
(I had to enter "U" to be offered US$ & "A" to be offered AUD$
Entering the same details as above, the Mastercard foreign exchange rate offered was:
1 USD = 1.320393 AUD
Now, if you Google for the daily foreign exchange rates offered by AMEX, you will not find them. This is very strange. I then found this on the Commonwealth Bank's own website:
https://www.commbank.com.au/support/faqs/1187.html
"You can view online conversion rate calculators set by the MasterCard and Visa card schemes.
While American Express does not publish an online calculator, rates for transactions in major currencies are comparable with other rates in the market. Cross rates can be provided for transactions. Please email American Express at [email protected] providing the date of transaction and the currency that the transaction was made in and American Express will respond within 2 business days."
What I did find was this website that compared the exchange rates offered by Visa, Mastercard & Amex over an entire year (with a substantiating graph):
Mastercard, Visa and Amex exchange rates - which wins?
The article is well worth reading, but based on the entire year studied, the result was:
(For US$ Conversions)
MASTERCARD WIN 21
VISA WIN 3
AMEX WIN 3
Next time I'm overseas I will use my Citi Plus Visa Card on the same day as my CBA Amex. That will be the only way to be sure (on that day) which card offered the better exchange rate.
The Citi Plus card is discussed in depth on the Whirlpool Forum as being one of the best to travel overseas with due to no foreign ATM fees (it is a debit card so no interest is charged when withdrawing cash) & no foreign exchange fees being charged by Citibank:
https://whirlpool.net.au/wiki/citibank_plus_transaction_account
https://forums.whirlpool.net.au/forum-replies.cfm?t=2616968