Note 037 From these sixty-six days, we must first deduct sixteen, as Pertinax was murdered on the 28th of March, and Severus most probably elected on the 13th of April (see Hist. August. p. 65, and Tillemont, Hist. des Empereurs, tom. iii. p. 393, Note 7). We cannot allow less than ten days after his election, to put a numerous army in motion. Forty days remain for this rapid march, and as we may compute about eight hundred miles from Rome to the neighbourhood of Vienna, the army of Severus marched twenty miles every day, without halt or intermission.

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Note to Chapter 5 of DECLINE & FALL by Gibbon