The parts that you changed should not give you worse gas mileage. Those things should give you much better gas mileage. That is a tuneup and those only help to give you better MPG. What grade of gas do you use. Are you driving it harder now than before thinking that it should go faster? Not sure why you would get worse mpg that sounds fishy. Just wondering, but when you replaced the timing belt, you made sure the the cam gears were in the right postion before you put it on? Bad timing can really affect a cars performance and give worse gas mileage. You can also take the car to Autozone or any parts superstore and they will read your ECU for any weird things if there are any. It is free so why not , as far as gaining power out of your engine. With the miles that this engine has, have a compression test or a leak down test to see how healthy the motor is. The b18c1 which is the Vtec motor compression is supposed to be around 180-200 not sure about the b18b but you can look that info up on the web, and a good leak down is no more than a 10% loss. Just some things to check before you actually decide to put money into this car. Have fun hope that this helps.