I've had my car a year almost and put 26k miles so far. Lately (past 2 weeks) i've been driving a lot less (only to work) because i've been riding my bike around.

My MPG per tank was around 450ish. Last 2 tanks, 350. I've driven the same and as of yesterday, my average was 30.2 then went to 29.8 within seconds (usually take quite a bit for the average to change so much) then it went to 29.3 and then turned into a roller coaster ranging .9 in either direction.

I have no idea what's going on. I wasn't sure if there's a cpu problem i'm not fully aware of. It's a '07 LS m5

 

I wouldn't panic yet foofoo, the mileage display on the DIC has to be taken with a grain of salt at best. The only way to get an accurate and exact mileage is to do it the old fashioned way - fuel receipts, calculator, logbook, etc.

As to why you've experienced a drop in mileage, there are a few things to suspect right off the bat.

1. The air filter, when was it changed last or has it been changed?

2. Tire pressures, checked 'em lately?

3. Plugs, how many miles on your HHR, if its close to 100k...change 'em.

4. Your driving style, since you've been riding your bike quite a bit, you are probably driving differently than you have been.

All the above being said, I wouldn't worry about a mileage variation so small.

 

im assuming you are doing a lot less highway driving and more around town. i too have a bike and when i ride the bike more my mpg average goes down. usually same as you if the weather is nice the car ony comes out for work in the morning.

 

I wouldn't worry about that...... the '07 mpg reading isn't "instantanious" in actuality, it's only good for an average reading, and even then it's off by a few mpg (most people have noted it reads less than actual) So it sounds like you are doing pretty good as far as mpg goes......

It's also possible some companies may be rolling out "winter gas" already because they can make a higher profit from it.

 

I tried looking at the air filter a couple weeks ago and couldnt get the cover off. What's with that? 6 little screws of hell. haha. I got the top 4 off and didnt realize there were 2 on bottom so I stopped. I need to get back to that. sooooooo much fun. hah

Tire pressure was checked yesterday before departing to cbus. I check it weekly.

about to turn 60k miles. and driving wise I have been taking it easy and shifting at 2k rpms. I was concerned with the mpg average jumping so quick. seconds instead of minutes of driving. it was...very odd.

i'm going to give a go at the air filter again. I almost want to get an after market so I dont have the horrible box anymore. haha

 

I would have gone with that but I went on a 1.5 hr drive the other day and it started acting weird. and the driving i do is typically 55mph or highway because I ride my bike around town and only take car for long trips (work is 15 min drive tues-thurs) and then cbus once a week. other than that. I dont drive. hmmmm fiddles. ahha

 

Extra long idling and/or more red lights do it also.

 

you do not by chance have new tires. I have noticed mine gets better mileage on the end of a tires life.

 

or maybe you are pushing the long pedal down farther.

 

Ethanol could be a factor. If all the stations in your area have not been using it, maybe you filled up with one that does? Also, most stations use "up to 10%" but maybe only 3% so if it is 10% that will make a noticeable difference in your mileage. I have noticed mileage decreases of about 10% when using ethanol laced fuels which is puzzling since it is only 10% ethanol but that is a measured and repeatable decrease in mileage.

Also, my mileage computer on my 08 HHR reads very optimistically. As much as 3 mpg higher than reality. My other GM vehicles are pretty close on actual mileage vs computer mileage but the HHR's computer lies.