This isn’t exactly true. In fact the alternator DOES produce “excess power”. It produces MORE energy that is used in MOST vehicles to 1) power lights, radios and so forth and 2) keep the battery charged completely. (Not all cars have high duty alternators).
Point here is there IS excess energy being produced from the alternator.
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The vast majority of cars have alternators that are computer controlled/on demand types that do not run continuously as the first generation ones in the 1960’s and 1970’s did ... They have very short duty cycles and high output.
If you want to save fuel ...
1.) reduce your miles driven if possible
2.) reduce friction losses , windows closed if at more than 40 mph , tires inflated properly , light engine oil ..
3.) lean out your mixture ,, most cars are about 20% richer than they should be for max mpg (rich for emissions) ,, retune using an exhaust gas temp guage and adjusting mixture by manipulating the O2 sensors signal.
I don’t think you are correct. Your statement violates the law of conservation of energy.
While the watts produced by the generator may be more than the system can use or store, the watts quantity transmitted through the belt to the alternator is greater by a significant margin. A 60 % effieiency would be good, meaning that there is a 40% loss.