my duster with 25" tall tires, stock 1bbl slant6, OD and 3.23 got it's best mileage at 60mph (about 30mpg) (about 1900 RPM). 65 it drops down to 27-28, 70MPH more like 25-26.
....and that's the "magic number". "Right around" 2000 RPM, give or take. It's a little different for every combination, but that's the general neighborhood.
