Spark plugs
 

If a car suggests say AP64 and you use AP63 ones would that be a heat range lower and get better MPG or worse?

Just curious...

Likely worse, a lower heat range plug is better if you are having engine detonation problems, I would think that the highest heat range that you can get away with would stay clean and perform best.

usual if your driving around like a grandma you would want a hotter plug