I was going to suggest a 240SX earlier as well, but for one it's not a new car, which was one of your requirements, and second, a 240SX probably isn't going to beat your 24mpg from your truck. I drove a 240SX that had around 40k on it for a while, and if you drive hard it can get as low at 19mpg. That little sucker will burn some fuel. The best I ever got was about 25, and it was a 5-speed. Oh, and the guy that owned it had the cat removed, and 2.5" exhaust on it, which should improve mileage. Hell, the 2.0/5-speed Probe I just got for a gas saver only gets 27-30 (30 on all highway). 24mpg is pretty freakin good. I'm not sure making a $10,000+ investment in an attempt to improve that would be a good decision. If your going for strictly gas saver, get an older 4 banger for $3k, or less. There's a lot of them out there, and you can find some in good shape. It will take you long enough to make that back with your 2-3mpg improvement as it is. If your just itching to get a new car, and are using gas mileage as an excuse, get what you really want!
Brandon