Someone did try a wheel/tire swap on a civic, and mileage went down.
The gears aren't easily exchanged. You'd have to swap the whole transmission. All the 92-95 generation (vx, cx = same good gear ratios) and 96+ generation (hx not as good as vx) transmissions are a direct fit, though.
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11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
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