To save gas, i reduced the rpm of my car engine, the problem is, the alternator is not charging the battery?

yeah put your RPM back where it belongs. your not saving that much gas to make a huge difference. your car is made to run at a certain RPM. if you want to save gas that much try some aftermarket parts. i gained about 15 HP and got an extra 4 MPG after spending about $300. well worth it

Run your car at factory spec for RPM (and everything else) and you should do fine. If you could get better mileage out of the engine/tranny setup in the car, the manufacturer would have figured that out before they sold the car. Don’t think that you can outsmart the manufacturer or get something for nothing. A more wide-open exhaust or intake, or a stronger spark from aftermarket parts might make a difference if you know what you’re doing.

If you are absolutely bent on keeping your RPMs down, you could mount a second alternator nd have all the power you need. However, the second alternator would be a drain on the engine. So, that kind of negates any benefit of lower RPM. To save gas, keep your tires inflated; make sure your air filter is clean; replace your fuel filter if its been a while or never been replaced; get regular oil changes, not just engine, but transmission and differentials too; and don’t drive aggressively, that means slow gradual take offs, and smooth driving, not hammering the throttle all the time. Those are the best ways to get you fuel economy as high as possible. Hope this helps.

you could change the gearing for the final drive ratio to lower you rpm’s while cruising on the highway to get better mpg’s
i plan to take out my 3.89 gears and put in some 3.66 gears