Higher mile 4hp30 525i like mine won't quite get 30 on the freeway any longer. Never chipped mine, kinda wish I had. I've got a 180k mile/300k km car and have kept an eye out around the central part of California. While the politics here are a joke, the cars are in really great shape. The SF Bay area cars and the SoCal E34s seem to have really high miles on them while the cars from Sacramento to Bakersfield much less so.
When I was shopping for my E34 back in the late '90's I drove only 525i and 535i cars and noticed on the on board computers that the city/highway cumulative MPG was in the high teens on 525i and about 16 for 535i models. I do think there is a bit of an mpg cost to displacement/hp. My E60 550i gets really great freeway mileage and really poor mileage in town with my foot in it all the time.
Link to my M50 engine rebuild thread
http://www.bimmernut.com/forum/showthread.php/43339-M50-Eng-4L30E-Trans-RRR-Far-More-Than-I-Ever-Expected?highlight=93+525+paul