I don't understand how your 06 matrix gets so poor gas mileage. I have a 04 matrix xr fwd manual with over 236,000 miles and I average about 42 mpg. I do drive on the highway 95% of the time at 65 mph and occasionally use the AC. I do have some minor mods. I installed a free flowing air intake system and a free flowing exhaust system with michelin hydroedge tires filled at 45 psi. I also use amsoil 0w-30 synthetic motor oil and lucas oil injector cleaner. This oil needs to be changed every 7500 miles which also saves some money. I use the injector cleaner every 1000 miles. I have been keeping records of my gas mileage since this car was brand new and i never dipped below 35mpg and that reading was done after driving in a snow storm. All this with regular gasoline. I don't know how you are driving your car but your mpg reading seem super low for your car.
I'm curious how you made out, if you SG still worked you could easily tell if the trans was locking or if you were staying in open loop.
As was said, get out on some flat highway (out of the area) and do some testing. There's probably a fuse or something you can pull to force the car to stay in open loop and you could do some runs that way and do some runs the 'stock' way, if they're the same then you're running in open loop.
Also make sure to accelerate as much above speed as you can before you start going up the hill (think about it as if you were bicycling), and definately come back down the hills in neutral (engine off if you can, but if traffic prohibits, neutral should be fine). I've also taken to coasting to a slow speed before I crest the hill and start accelerating down it, like I'll drop down to 45-50 and accelerate in EOC to 55-60 while going down the hill. You might want to leave your engine on to maintain power brakes if your hill is very very steep.
it's where i live (IMO). i live in a basin, so where ever i go, i have to drive on steep hills. The 2700 + 150 pounds + ridiculously steep hills = crappy gas mileage. I never floor it/ i have a gutted air intake box which i forgot to mention (maybe i shouldn't have done that), DIY ground wires, and 45 psi tires on my stock 16 inch rims. I can't find a 00-03 Golf tdi 4door 5speed anywhere here in Cali... I'm royally screwed.
VW's rarely score well in ConsumerReports...they're dogs....consider another Japanese model?
It's got to be the car or the driver. I own a 2006 Vibe with AWD and as we all know it's the same car as the Matrix and I have pounded the car and still manage to get 26-27mpg in the city. If I granny it i.e. not getting above 2500-3000rpms I have gotten 26-30mpg in the city.On the highway with the air conditioning on I have hit 33-34mpg. I do have a CAI on it and run mobil 1 5w-30 syn. I do not OVER inflate my tires, I run them at factory spec. I also run a scanguage II and my daily average is 29-30mpg.
From the comparative experiences of Matrices in this thread, I'd say the single biggest obstacle to getting better FE is that automatic tranny. For so many of my friends and relatives over the years, investing 30 minutes for training in driving a stick has been a no-brainer. There's been no looking back: None of them have switched back yet.
As for other choices of similar small cars, the Japanese and Korean models still beat the best of Europe in reliability. I recently compared the Mazda5 and Matrix was surprized how similar they are (Mazda5 being a little bigger and the Matrix having a little better FE).
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein