I can't believe no one has asked about this yet...here goes. I am working with a '98 323is high mileage vehicle with good maintenance schedule. I have gone from 24-27 MPG a couple months ago to now around 19-20 MPG. This is NOT correct and I do not know why. Can someone help? O2 sensors good, no codes, stock air induction (want to change eventually), cylinder compression good and consistent, no fluid leaks, newer plugs, newer complete cooling system overhaul (fluid, pump, leaks), new fuel filter, did not run fuel system cleaner yet... winter months have begun here in WI...a factor w/ different gas formulations at the pump? possible bad wheel bearing up front - does this factor?...running on winter tires (Dunlop Wintersport D3) with a minor bent rim - does this factor? ...auto trans needs fluid/filter replace - does this factor? Why am I losing 5 mpg?
Lots of possibilities, seems to me, including some of what you mention. Not sure to what degree it all might add up to, though. Different gas - yeah maybe, but I wouldn't expect much difference from that necessarily. Winter tires - yes, if you think it's likely they might have greater rolling resistance, which might be possible from the tread compound. Any difference in the winter/summer tire sizes, and how are you calculating your mpg? Have you compared what the computer is saying vs. tracking your mileage and gallons used per tank? If you have a substantially different rolling diameter, that will make a difference. Cold weather running might make some difference - less efficient until things are warmed up, although if trans & diff fluids aren't getting up to the same temps as when it was warmer, perhaps a bit of viscosity-induced drag there (although, I still wouldn't expect much). Bad wheel bearing - sure, as it could be creating some drag that's taking more power to overcome. Are you sure you don't have any vacuum leaks? What about a possibility of vanos not functioning correctly and costing you mpg's?
