Milage increase I don't know. However in my 422 ci duster 3500#with manual 5 speed od it would get up to 22 23 mpg on the highyway at 70 75 mph. This is with 34 ° maechanical and 12° vacuum for total of 46° Total timing. This all being very conscious of milage! But even when driving spirited and careful of the law 18 19mpg was the norm. Gears are same as yours and tires are very close 275 60 15 BFG drag T/As. Rpms at 70 is 1700 in OD