I think what's happening here is that is what the car rental companies are selling you pesos at an inflated rate, which is not a good deal. For example, let's say you have a rental car bill that is exactly $200 USD. They will write the contract up and charge your credit card $4,000 pesos (200 * 20). Now when that charge is put through on your credit card, the bank isn't going to give you that much, and at $17.50:1, what you will see on your credit card is $228.57 USD (4,000 / 17.5).
If you can, use the car rental company's Mexican site so the quote is in pesos to begin with.
As far as I can tell, Capital One uses the daily peso exchange rate - not some made up number. We get charged very very close to what we are quoted, realize there's 7 days or the rental time between counter quote and actual charge - they charge your card when you return it.
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