My girlfriend has an '85 528e with 178K miles on it. She has noticed pretty low gas mileage since she got it (a few months ago). 20-21 mpg is pretty standard. I have also noticed a fuel smell in the trunk after driving. Looking underneath the car the tank doesn't seemed to be rusted at all and I don't think the fuel filler neck is either because it smells the same no matter how full the tank is (plus it was an Arizona car). I have heard the EVAP canister can cause this smell when it malfunctions but can this also be causing the low gas mileage? I have looked around and read that 25-27 mpg is pretty common with these things and she does a mixture of mostly highway and some city driving. Any ideas?

im waiting for the answer on this i have the same problem

Your car is 20+ years old. So it's probably safe to say the car needs a general tuneup. You provide no details about recent maintenance, so this is worth mentioning. Cap, rotor, spark plugs, ignition wires, intake boots, O2 sensor, etc. They are all consumables and have been neglected on the typical E28. Items like your fuel injectors, throttle body, and such likely need to be cleaned. Valve adjustment--how long ago? But even then it will probably not run at the same efficiency as a new E28. 5 spd vs automatic also comes into play with regard to fuel economy.

About the fuel smell, you should check for leaking fuel lines in the engine compartment and inside the trunk. Or just simply replace them all should they be original. If you remove the carpet you will see various hoses running from the fuel tank to the plastic tank by the strut tower. Let your nose take you to the source! Also, the gasket that seals the fuel pump/sender assembly at the top of the gas tank may not be working. Remove the black access panel from the trunk floor to look. The fuel tank filler neck is highly susceptible to collecting dirt and debris kicked up by the rear wheels, so just because the car came from Arizona is no guarantee there aren't tiny rust holes from that gunk trapping in moisture over 20 years.

A good tuneup as mentioned with OE Bosch plugs, a new o2 sensor as well as replace your fuel filter will help alot. As for the fuel smell in the trunk is could very well be the evap tank and associated hoses or it could also be a seeping sender/pump acess cover gasket. You can check these by removing the access panel under the carpet in the trunk and check for any seepage. Its not uncommon for this to happen as they do dry out and begin to leak. With that being said, this would be perfect time to check out all of your fuel lines.

I often see fuel filter mentioned on these bad mpg threads, but shouldn't a clogged fuel filter actually deprive the car of fuel rather than make it use more?