2005 Aveo 5 poor mileage

I see the graph of mileage vs speed, looks right. This is my first GM product in 30 years. I had a 2001 Suzuki Swift 1.3 litre 5 speed. it would get 45 -48 highway MPG. The Aveo is lucky to get 27-30. The RPM at 70 MPH is 3400. Is there a different year or vehicle that I could swap the differential or transmission 5th gear with, to drop it to 2500. The engine has plenty of power at 2500 to run the car along. Anybody put a K&N filter on, or a low restriction muffler with positive results. Any help would be appreciated. I like the car, but highway fuel economy is terrible for such a small car. Thanks Stephen

I'm a new guy here but it sounds like your car is not shifting beyond 3rd gear (I had the same prob) In my case a re-learn procedure on the crankshaft position sensor cured it. Edit...I'm assuming you have auto trans.

Mine got 27 MPG hwy brand new in the summer time....does 28 now, with synthetic fluids/K&N filter/Iridium plugs/new 2012 throttle body/most recent PCM software updates installed and no a/c on....and both those figures are with premium fuel. When it's cold, 21 mpg. I did all this in the past 4 or 5 months was getting 23-24 mpg prior...I did get 31 once this summer on a tank right after all the above was just done.

Seems to me whatever I do the computer changes something to make it go back down LOL I should maybe change the other O2 sensor see what happens.

you could get a diesel ? i think ? vauxhall f15? tranny in there. I forget the specs but there is one that has a taller final drive and an equally tall 5th gear. As well as one that has a taller 5th gear and an equal final drive (if thats your preference).