What kinda mileage are you getting currently? The HF is a great platform in itself (though the power makes it difficult compared to the z1 for the average driver).
My best tank so far was in the low 70's and worst was in the 50's. I would say if I ragged on it all the time, I could get it down into the high 30's. I also think it has the potential to get over 100mpg.