The new stage comprises elements from Guanajuatito, Derramadero and El Brinco, as predicted by Autosport's sister publication Motorsport News in September. It will run as SS20, at 0833 on the final morning.
Crews will then be permitted a change of tyres before a superspecial stage and the 10-mile powerstage.
A spokesman from Rally Mexico said: "After working for years, and gradually building up our capabilities, we've finally achieved a long-coveted milestone: the 80-kilometre stage.
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"The [stage is the] maximum length allowed by WRC regulations, and 25km [15 miles] more than our longest stage of 2015 — through the mountains of Guanajuato.
"It will put every resource to the test. Every drop of mental, emotional, and physical strength from driver, co-driver, car, tyres, and team will have to deal with about 50 sustained minutes of intense competition, a feat of concentration and exhaustion that hasn't been seen in our sport in decades.
"We have no doubt that this news will put a small frown on the face of some, but we're sure they'll put a wide grin on many others. People like us."
The previous longest WRC stage was the 51.57-mile Liamone-Suaricchio on the 1986 Tour of Corsica.
Such long stages were not unusual in the 1970s and '80s, but a move to shorter tests was part of a raft of regulation changes brought in with the end of Group B in 1987.
When Autosport put the idea of the 50-mile stage to Sebastien Ogier earlier this year, three-time Mexico winner said: "Why not? It can be hard for the tyre, but my record is quite good for looking after the tyres - so maybe this can be good for me. It would be quite tough."
Rally Mexico provided this season's longest stage of the year, Guanajuatito at 34 miles, but just missed out on the longest total distance - at 244.96 miles, it was 0.26 miles shorter than Rally Italy.
The 2016 event will again be the third round of the series, and runs from March 4-7.
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