[70AARCUDA]

All anecdotal, I know, but from personal experience:

My 1984 Honda 1.6L DX got 40-42 MPG with A/C on the highway, but todays' equivalent model doesn't get anywhere close to that. Tighter and tighter Emissions requirements and more and more mandated Safety equipment have combined to increase vehicle weights dramatically!

My 1970 AAR 'Cuda with 340 cid V8 engine had a curb weight of 3200 lbs; our 2011 "compact" Cruze LTZ weighs more!

 

[70AARCUDA]

you cant have stricter emissions and more mpg, it will never work.

...exactly! In fact, in order to meet the stricter emissions requirements, all engines must actually burn MORE gasoline than necessary, especially during cold-starts, so as to "burn-off" everything inside the catalytic convertor(s). And, this "richer-than-necessary" condition also occurs (but to a much lesser degree) when the engine is warmed up and running in "closed-loop" mode under control of the engine computer module (ECM). It's happening ALL the time!

...simply put: CLEAN AIR = LESS MPG than optimally normal.