your milage may vary.
as for the sumis, i love them! lots of grip, the car is very stable on the highway, unlike the oem tires. but even though they are considered lrr tires, i am still giving up about 12% (8mpg) compared to the oem re92 potenzas.
however, my last tank was 78.2 mpg, and thats using 10% ethanol (3% energy loss). not too bad, considering.

2000 Honda Insight. Kenwood MP3 head unit, Blaupunkt door speakers, dual Cobalt amps, 3 MTX stealth subs, 4th very-high-mounted brake light, Sumitomo tires, yellow-top, hot induction air mod, tinted, SG2, MIMA, center armrest.

best tank 82 mpg