Our 2009 Aveo5 is only a few months old with less than 2000 miles on it. We like the car very much except that the city MPG has been disappointing. The city EPA estimate is 25. The best we have been able to get is 22 MPG. After installing a scan gauge and driving very conservatively I was determined to find out specifically why the fuel economy is so inadequate.
After considerable research I found the transmission is made by Aisin, a Japanese manufacturer with a good reputation for quality transmissions. It is an ordinary 4 speed, with no overdrive. The same transmission is found on all Aveos made from 2004 to 2010. I recently found the service manual for this transaxle, Aisin 81-40LE. It can be found on-line at: http://www.anagabriela.ro/.../2.%20AISI ... NSAXLE.pdf
The graph on page 5A2-12 shows the manufacturer’s specification for shift points. (The graph is stated in KMH, so I’ve converted the figures to MPH for this discussion.) The graph shows the shift points will vary somewhat according to the TPS (throttle position) reading. On my car a scan gauge registers the TPS reading at idle at about 16; very conservative (slow) acceleration would be about 25; brisk or above average acceleration would be about 35 TPS.
According to the graph, at 25 TPS the shift points should be at about 11, 20 and 27 MPH. At 35 TPS it should shift at about 12, 22, and 32 MPH. I compared what I am actually getting on my car, at about 35 TPS, which would be considered average or brisk acceleration. I found it actually shifts at about 15, 25, and 45 MPH. In other words, the upshift into the highest gear (fourth) is much too late. Most 4 speed transmissions typically shift at approximately 10, 20 and 30 MPH. The delayed shift into high gear at around 45 is a major reason for delivering poor city FE. I live in suburbia. On major roads, typically the speed limit is 40. Unfortunately, when there is traffic it is impossible to break 45 – thus the car is spending most of its time in first, second and third gears at best, and it virtually never goes into high gear, which would yield the best fuel efficiency.
The transmission shift points are entirely determined by an electronic module called the transmission control module (TCM) which was designed and pre-programmed by Chevrolet. The shift points for which it is programmed are widely different than the documented specifications of the manufacturer, Aisin. I visited a Chevrolet dealer and spoke with the service manager to see if the TCM could be reprogrammed. He told me that they could do nothing to change it, unless Chevrolet had issued a service bulletin update for the program (which is has not done in the past five years that Chevy has been using this transmission on the Aveo). In other words, unless and until Chevy engineering rewrites the programming of the shift points and offers it as an update for dealers to install, nothing can be done to rectify the existing discrepancy. I then visited my local transmission shop (AAMCO) and asked the same questions. They confirmed that there is no way to reprogram the TCM unless Chevrolet rewrites the programming for it.
From what I have read on the web this is a very common complaint for Aveos. Most owners are unaware that their cars are late in shifting into fourth gear and frustrated with their poor city mileage. Seeing that there is documentation supporting the fact that the TCM programming of Chevrolet is greatly divergent from the specifications of Aisin, the manufacturer of the transmission, I believe Aveo owners have a valid grievance against GM and should press to have the situation rectified.
Who can we complain to? How can we unite to complain effectively and have Chevrolet correct this situation?