2008 M35x Only Getting 10 MPG

[alskie]

I recently had my ECU replaced. Since replacing the ECU/ECM I have noticed that I am only getting 10 mpg. Before replacing the ECU/ECM I would average about 18 mpg. I have reset the ECU/ECM twice. I would appreciate any advice on what I can do to resolve this issue. Thanks

[Ilya]

How have you reset it? Using the reset procedure or with a reader? Have you tried disconnecting the battery for many hours overnight?

[alskie]

I disconnected the battery for 12 hours.

[alskie]

I mistakenly reported that this problem started happening when I replaced the ECU. I forgot to indicate that I had new tires put on, highspeed balance, and a wheel alignment done. That is when I noticed that my car was only getting 10mpg. I would appreciate it if someone could advise of what I can do to resolve this issue.

[VStar650CL]

One way to inculpate or exculpate the tires is to way overfill them and see if it makes a big difference in your mileage. If it does then the tires you chose may be way stickier than what you want for gas mileage. Performance cars like your M also tend to have exaggerated negative camber and front toe-in for handling, but if you don't routinely hang the a$$ out in the corners, those things are also bad for mileage because they make the front tires deform and scrub when cruising straight lines. Going slightly positive and out from spec in front will help, but go slightly toe-in from spec on the rears to compensate so she won't wander at high speed.