Its because the car from factory will generally run too much fuel to ensure that all possibilities are accounted for. Most aftermarket ECU's will lean out most area's the major one being part throttle application where you spend the most time (cruise is generally fairly lean anyway on most modern cars), this is why you will see a difference in economy. As to using more fuel to make a bigger bang, that's not technically correct, different engines will make more power on different air to fuel ratios. On a turbo car the more fuel can have a cooling effect on the exhaust charge (it can also, depending on the timing have a positive effect e.g. anti lag applications and earlier spooling of the turbo via retarded timing and increased fuel usage) which will mean the turbo will not spool as quickly as cooler exhaust gases will generally move slower creating pre-turbo back-pressure, less scaveging of exhaust gases etc (this is just a rough example btw before techincal purists jump all over me). Snappertom, sad to see you move away from the rexie but I think you've made a good choice with the T. I know its where I'll be heading, except with the retention of the rexie as well... Cheers Brett