I have a mildly tuned engine with cams, manifolds, sports cat, exhaust, the usual list and I havent found my economy any worse that it was standard, honestly. So from a fuel bill viewpoint tuning more conventional methods shouldnt be ruled out. Obviously you know that this is an expensive route compared to the "quick fix" of NOS, that I guess will give you a good boost on the odd ocasion you need it. I'd love to have a go of NOS myself just out of curiosity and an easy 50bhp boost sounds great, but regardless of the retailers asurances that "set up properly" its perfectly safe, you just read to many cases of engine failure and premature wear assosiated to it use. For that reason I wont try it, and also because carrying round a big explosive bottle in the car gives me the creaps, as to does your insurance company, and then theres the cost of refils, and now it dosent look as cheap as first thought.