Automotive → Re: Why better MPG at 80 vs 70?

you got something backwards, more dense air more fuel required to get proper mixture rate.

hence, intercoolers.

But that also equates to less throttle to maintain a set speed.

Yes but speeding back up again uses more gas than holding a set speed. Engines are most efficient when running at a constant and stable cruising speed.

my 2000 impala sits around 2300rpm I think at 80 on flat road with the overdrive kicked in.

I know this but what does it have to do with my post?