Wannabe hypermiler here. I'm never buying a hybrid since I think it's full of shit to load half a ton of batteries into a car to improve fuel efficiency. I'm instead taking the route of biofuels and getting a diesel Mercedes.

For the best results, I've narrowed down my search to a 4-speed manual 240D. They're known for being reliable, efficient, comfortable, and few standard electrical systems to go wrong, so long-term maintenance will be much cheaper than other Mercedes cars. However, it leaves one thing to be desired: its 65-horsepower engine, which in all reality, most of them are probably down to 50 now. Normally, one would just grab a turbo and slap it right on, but since I'm wanting to go hypermiling with this car, that's not an option.

I have a gasoline-powered Saturn, and I've upgraded its intake and put in a high-voltage ignition system, both of which have got it up to 45MPG highway. Since the 240D can only take the former upgrade, is there anything else I can do to improve its performance without compromising on fuel efficiency?