What mileage does your slant six powered ride get?

My Slant running on a good day gets around 18 to 22, w/ Mopar elect. ign., Super Six mod, headers, dual exhaust. 904 Auto. and 3.23 gearset in an 8.75 center chunk. 87 at the pump.
1961 Lancer
A904
2.73 rear
Head work, mild cam, Clifford intake and shorty headers, Holley 390

24mpg highway, 65mph

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What size tires guys?