It's not a big deal. If the exchange rate were 16.41:1, you wouldn't expect the cab driver to get out a calculator and tell you that the 85 peso cab ride will be US$5.18 and give you US$4.82 in change for your 10 dollar bill. By the same token you can't expect the driver to charge you US$5 and take the loss.Thats the rate they use. Doesnt matter if its 16-1 or 14-1. Its the rate they were accustomed to getting last year......
The fares are set in pesos; cab drivers figure, and rightly so, that if you are going to pay in US dollars, rounding the fare up to a convenient number for them isn't going to be a hardship for you. If you are feeling "ripped off" by this, pay in pesos.