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Having scene students make bad assumptions over the years always makes me bit weary of results pulled from "simple" math by plugging numbers into equations. Take no offense on that one :-) There's enough energy in a gumball to power the world's demand for electricity, if you think can split the atoms apart.
As to your request it goes back to alot of variables. It's going to differ on the same car based on even simple things like auto/manual trans, and final drive ratio. Every engine has a different rpm when it operates most efficiently, and depending on your setup, the amount of power produced at the sweet spot and the gray area around it are going to have positive or non-noticeable effects from weight reduction. If the car is underpowered for the sweet spot of the motor, say an old jeep motor from WWII at 40HP trying to drive down the road. Yeah on level road, minimum road the 40hp will get you to 50-60mph, but an escort of similar mass with 110hp will do faster and use less gas. And how FE and mass plays into that situation: If you add 4 soldiers to the jeep and 4 people to the escort, which one struggles more to move down the road? Obviously the Jeep. If the power/weight becomes extreme if you need to run the engine under more of a load, and thus more fuel.
Now if you have a vehicle, say something like a v6 or v8 pickup, the amount of power and torque produced on the lower-end rpm yields a power/weight excessive enough that removing weight will not have an effect. But if you take something with a smaller engine that produces 60bhp at 1500-2000rpm range then the power/weight gains could be made with weight reduction.
I remember a few years ago there was a video where someone started cutting apart a small sedan (maybe Nissan Altima?) and the zero to sixty time went from 10 sec to 5-6second range. Obviously they did a bit of damage to aero without a hood, roof, doors, etc BUT the point is that the weight reduction made the car faster to get up to speed. And that helps alot depending on where you're driving as well. If the commute is alot of stop and go with stop signs every block or few miles on back road, then the potential to gain some FE by getting to speed sooner. And that will be true for the pedal to the floor or a conservative feather-light throttle.
Hopefully that helps a bit.
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