Yes, improper inflated tire will cause you mileage for sure. I have done this before. My tires were down abou 3psi, and my fuel economy dropped about 1mpg. Once I got back to 33 or 34psi, then I can start to get around 19.8 to 20 mpg.
While mileage is admittedly a few mpg less than what it should be, that's not something I would consider to be indicative of a bad car. My mileage is all local, by the way. On freeway driving, I get about 23-24 mpg.
I don't doubt the HP or mileage figures you quote for the SS's and I guess that if I had a mullet, tattoo and a copy of an Iron Maiden CD it would be a great car. The one thing you have to remember, it is hard to get any kind of gas mileage or win any races when you are sitting on the side of the road brokendown. I might not be faster, but I don't need AAA either.
01 Silver IS 300
01 Toyota Sequoia Limited
Why all the complaining about gas? Is it much lower than the average automatic inline 6 RWD sedan? What is the avg mpg for a sedan of the IS' caliber anyways? When driven like normal human beings, not pretending to be speed racer. 20 mpg isn't that bad IMHO. My 91 camry was ~23, 89 prelude ~20. The 1st car I ever drove was a 79 Lincoln Continental and that had the same mpg as a lambo, 9 miles per f*cking gallon. So why all the fuss over 5 extra bucks a week?
------------------ An Ecstatic Owner of a Fully-Loaded Graphite Gray 2001 Lexus IS 300