I cleaned up my grounds, got new posts and now my dash is reading 14-15 while running and before it was at under 12 . Is it possible to increase that much just from cleaning grounds and new posts?

 

mine reads 13 and I have no qualms with it? lol Besides that. A vehicle is best to have around 13 volts rather than the spec 12v due to the fact that it has something to compensate for whenever something comes on and it goes under load. Basically it wont kill everything. But no your car is fine lol

And yes it is possible if you had a lot of corrosive material build up around them. That would hinder your electrical system.

 

Can make a giant difference if something was not making a good connection to ground. If you had under 12V something was wrong. I'm not happy unless I get 14v when the car is running and no electrical components are on. If you still have the original battery cables you might get a little more power if you change them also.

 

I think around 14.4v is optimal, so good job. Cleaning up grounds really does help!