Former Chrysler service advisor here!
I'm not sure of the total diameter, but it will be pretty close - usually bigger rims get tires with smaller sidewalls, so it usually balances out.
If it's off however, the service department can recalibrate the speedometer using a Chrysler service scan tool.
And I agree you can try asking them to swap out the 20's with another car, or they can just order a set of 20's. You'll pay for them of course, but they'll usually cut you a deal pretty close to invoice.
Good luck!