I don'd think any additive,by itself, is going to increase fuel economy. Certainly, if the engine is not running as it is supposed to, due to dirty fuel injectors, or if carb'd, if say it has a sticky choke causing it to run rich, then additives to help correct the problem might/should help, getting the engine to run as it should.
I think the basics are best followed; proper tire pressure, engine in good tune, etc. Some tricks that should help would be a low restriction air cleaner/ intake system, a less restrictive exhaust system. Make the engine work as little as possible by not packing around a bunch of dead weight in trunk/ bed. Keep the speed down ( pushing air goes up by the square ). The old "egg under the gas pedal" attitude.
Most all cars & trucks built in the last 20 years have been designed, by necessity, for best possible fuel economy. One very little item often overlooked is the air dam under the front bumper, often torn off by accident. They really help re-direct air at speed, keeping it from creating drag...if yours is missing, get a replacement.
Mark