If you want ultimatel mileage you want to stay with the OEM tires as they are designed for low rolling resistance. I own a 2000 Insight and my current LMPG is 77.7 but I am still learning and improving. I've had several trips over 100mpg with most door to door commutes averaging in the 80s. It's a 5 speed and has the warm-air modification, engine block heater and I try to use all the hypermiling tricks I can including running my tire pressure around 60psi.

Just keep researching and enjoy.