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Originally Posted by 19bonestock88
Well, my car has the 4T45E transaxle(still despise it's being an automatic but no manual was offered on this car) with the 3.63 final drive ratio, and I turn around 2600 at an indicated 80mph... Other final drives were offered (the 4T45 behind the 3.5V6 was 3.08, and the Malibu Maxx had like a 3.23 ratio to compensate for weight), but I don't know how well my L61 (2.2 L4)PCM would play with a taller transmission... Not that it has power to turn taller gears anyway... The main issues with getting great highway mpg is the interstate on which I do much of my highway driving(most of it is two lane at 55-60mph), it has lots of 5 and 6% grades, on which the engine struggles, and the trans kicks down, and then I'm turning 4000+RPM getting an instant reading of about 12... Figuring out how to get the trans to stay in OD and still maintain speed is just a matter of reducing resistances(drag, rolling resistance, weight), but I'm just getting started...
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With HPTuners you can command the transmission to shift faster (for less time spent shifting, spinning and creating friction) and cater your shift points to how you want. You can have the transmission shift at certain TPS%, load, speed and pressures. You can fine tune the TCC lockup points to command to stay on for longer or early, so you can stay in OD longer. It's not something anyone can do and it's something I've rarely messed with, but it can be done by a skilled tuner. I'm just a novice tuner at best.
Depending on ECU, you might have access to it's lean burn modes, deceleration fuel cut off points, you can command when the drive by wire throttle hits PE mode (power enrichment, which adds extra fuel to prevent lean conditions on sudden acceleration), some older models can have the O2 sensor trigger/switch points adjusted to keep the engine running leaner for better economy (doesn't seem to work with my 09 Sierra, but it did with my 02 Silverado, for example).
Even a proper adjustment to the VE and MAF tables to follow the true curve of the airflow coming into the engine can improve power delivery and efficiency, when you're commanding the perfect AFR for your driving conditions. Monitoring AFR with a wideband is definitely needed to start running lean like that, but it's all possible with a tune. You're not going to magically get 3-4MPG, but I can see a bunch of these changes squeezing 1-2MPG out of most engines, especially GM's. They are generally tuned very conservatively to make sure they can run on the worst kinds of fuels in all weather conditions in various states of disrepair. This means they run kind of rich by default, it's safer for everyone, but is not ideal for performance and economy.