McGarrett's 1974 Mercury Marquis Brougham is seen in "A Gun for McGarrett" (Season 7).
Did you ever wonder what kind of mileage McGarrett got on the big Mercury he drove? Well, someone worked the numbers and learned that the Lincoln Continental, which had the same body style as the Mercury Marquis, achieved only 7 miles per gallon. Whether that was in the city or on the highway was not specified. In either case, it seems likely that McGarrett was very glad that the state was paying his gas bills.
Word is that Jack's Sedan DeVille received 5 miles per gallon. Probably, it was in the same 5-7 mpg range as the Lincoln and Mercury.