I put a TBS from Rusty's (see the pic below...) on my 2000 XJ. I also put the intake kit from them as well thatcomes with the K&N filter on. I think that I MAY have gotten an extra 2 mpg out of it. What I did notice though, is that the gas pedal is quite a bit touchier now. It has a little more raw power under the hood than before. I know I got a little worse gas mileage at first because I kept stepping on it to hear the air filter and intake hiss and whistle away. I would say that it is worth doing it, but don't expect to see a big change in you mpg.