would adding a rear diffuser improve fuel economy to a car ? or would it lower the fuel economy by creating down force ?

 

Unless your commute consist of you doing sustained triple digit speeds for a extended period of time, i don’t think your going to see any difference in MPG.

 

More drag, so more consumption I think.

 

Rear diffusers don't create drag (if implemented properly). On the contrary, the job of a rear diffuser is to accelerate the low pressure air, exiting the rear of the vehicle to create more downforce (negative lift, blah blah). The OEM bumper, unmodified, creates a lot of drag by parachuting the air in the "pocket" towards the bottom, a diffuser eliminates that, so in effect, it helps reduce drag immensely for the s2000. But just as Junky said... You're not gonna really notice fuel economy benefits unless you're at a constant high speed pace. You don't happen to live in Germany, do you? lol.