So my trip computer is telling me that I am getting 27 MPG in overall mixed driving conditions. We all know that this is impossible but, of course, my dealer has checked it twice and tells me that the computer is fucntioning properly. Even more odd, every now and then I will start my car and the MPG has gone from 27 (with an average speed of 31) down to 10 MPG!

Anyone else having this type of problem??

 

Yes. It has happened to me a couple times now in my 08 type S 6MT. Just yesteday I had the trip computer screen on. Showed I was getting avg 24 mpg, avg 31 mph, drove 160 miles, 5.5 hours. I shut car off. About an hour later, I got back in the car and it said 0 mpg, 0 mph, 160miles, 5.5 hours. So the mpg and mph zeroed out!

I just looked in the owners guide and I did not see anything that suggests this is normal.

Anyone know if this is normal (eg, maybe at 1/2 tank it rechecks calcs?), or a problem?

 

^ This does not seem right unless maybe your ignition was left on on a long-shot hunch. You may want to try this to trouble-shoot:

1. Before starting car, move key to Acc #2 position so your computer comes on. Check the values then- hopefully they're consistent with what you remember before shutting down the engine
2. Next start the car and see if the values reset? If they reset, I would start suspecting the battery. Seems like other stuff would affected if it were a battery though

My trip computer has been dead on my entire time- over 3 years. Never has it zero'd out mid tank. The computer continuously updates the MPG but you don't see much change by mid-tank because the MPG only moves in 1 MPG units and your average has converged. I have found that after 3/4 tank my average mpg consistent unless I hit the highway for a long trip. My average speed is usually 26MPH and the MPG is 18-19.

I just checked my records on www.fueleconomy.gov and noticed that at 31mph, I routinely get 21-22 so you guys getting 24 is pretty plausible. I would classify myself as a spirited driver.

 

Thanks. I had seen another thread about this topic last night. Tried to locate via search now, and haven't found yet. My guess is most don't bother with the MID trip computer. Either more often using MID temp gauge, the navi trip computer, or Navi trip computer hack (ie, even if it is happening more frequently,people either don't see it or don'tcare).

Anyway, it is no big deal. I calculate my mileage in a logbook using odometer so I don't rely on the thing anyway. But I am more concerned that it may be a symptom of another (larger / electrical) issue. You noted the battery, for example. I have had intermittent issues with the memory seat function not working. I had been thru the forum and some TSBs and brought issue to my dealer. They could not reproduce the problem (when do we ever get so lucky), but they said the battery had a weak cell so they replaced it. I found no signs that they really had (think they were BSing me), and the bill (n/c warranty) did not list it until I pressed further- then they listed replaced battery on a reprinted bill.

The "battery fix" was sometime ago, and I had not really paid attention to the MID trip computer but just happened upon it and noticed recurrence of mpg /mph reset only. And as you noted, battery probs would cause more than mpg/mph MID reset and leave rest alone.

 

^ glad to help. If you suspect the battery, a battery tester that plugs into the cig. lighter can conveniently check the voltage when you start the engine. There's a thread about evaluating your battery's health- I remember about 12.1-2 volts initial voltage overnight and a drop no less than 9.6v while cranking the motor. I just changed my battery last month as a precaution. A Costco battery is just $65 and it is much more powerful than OEM.