How can you improve gas mileage for a 1988 BMW 735I?

Well it all really comes back to one thing, you can't punch it all of the time I know that might seem hard not to do but it's the only way that you can get "Good" gas mileage. Make sure that you keep the Transmission setting in the "Economy" mode (Represented by an "E" on the dash display). Keep the transmission from shifting at more than 2000 RPM. Use the cruise control a good majority of the time. Keep tire pressure at around 37 PSI. Modify the air in take with either a K&N or a Stainless Steel Spectre air filter. Make a "Make shift" heat shield, to block manifold heat out of the air intake. You can use the bottom half of the stock air box. Avoid unnecessary up shifts (i.e. going up hill). Keep the car running at tip top shape. Hope that this will help you!