Praise be to Allah, the Lord of the Worlds.
According to Shafie jurists (May their souls rest in peace), the distance for which a traveler is allowed to shorten prayer is forty eight miles. They have based their opinion on the following narration: "Abdullah Bin Omar and Abdullah Bin Abbas (May Allah be pleased with them) used to offer the four-unit- prayer as two Rakahs, in addition to breaking fast whenever they travelled for a distance of four bourads or more (forty eight miles)." {Al-Sunnan Al-Kubra}.
Four burads equals forty-eight miles, but there was disagreement about the mile. According to [Al-Minhaj] by An-Nawawi, and the commentaries of scholarly books, a mile equals six thousand cubits. However, early scholars said that it is three thousand and five hundred cubits, according to the more correct opinion, as stated by the Shafie scholar Al-Samhoodi in his book, [Waf` al-Wafa` Bi Akhbar Dar Al-Mustafa."] This is also the reliable estimation of Al-Samhoodi in his book, [Boghyat Al-Mostarshideen, 155-156.]
It was also adopted by Dr. Mohammad Al-Kurdi in his Book [Al-Makadeer Al-Shariah, 2525-260). For further details, kindly check this book for it covers many problematic questions.. Al-Kurdi clarified that the cubit, according to Sharia, equals forty-eight centimeters. In conclusion, the distance for shortening prayer equals (80.64 km), and this has been adopted by most modern scholars. And Allah knows best.