[Fat Charlie]

Tank over tank results in varying conditions just aren't the same as instantaneous mpg and gph results.

With a gauge you can learn on one commute what would otherwise take testing over several tanks to establish and memorize. With a gauge once that's done you'll also be better able to adjust to changing conditions instead of just knowing what your car will do on a pool table. Besides, the distances are far enough means that minor differences in results add up quickly- all the more reason to have instant data!