weight, resistance, and driving style are the easiest to explain. HP or efficiency is more complex.

The lighter the car the better. The less friction you create, the better. The more you hypermile the better.

lightening the car means eliminate everything you don't need, want, etc. Including the driver.

Resistance can be lighter wheels, smaller width tire with a low resistance tire. Increasing the tire diameter with le width and weight will help you go further. BUT you must realize this in the on board gauges etc. If you increase the tire diameter, your speedo and odometer will be inaccurate to the difference in the circumference of the tire.

hypermiling is different for the car you drive, where you drive, and the traffic your in. Do your best, drive furthest locations for warm up, look at the fuel maps for optimal gearing and rpm ranges etc.

HP and efficiency is an odd balance. if you think of the engine as an air pump, the more air you can pump through it the better the hp.. more air = more fuel. However, if you look at the modifications you have done, and the new fuel trims etc, to determine the new optimal driving style. You should be capable of doing better than stock.

Gearing is key with hp.. if you have enough hp to drive in 5th gear at 1500 rpm versus 4th gear at 2500 rpm, you will save fuel provided you are using the same fueling. Problem is, being able to make it up a hill in vermont at 1500 rpm in 5h gear. (im not even sure how fast 1500 rpm in 5th gear is, but im making an example.)