The best place to start is probably with the following article:
Achieving High Fuel Efficiency
As for the weight of the spare, it does technically matter, but not nearly as much as your driving techniques. Several members here who are experts at high efficiency driving can exceed EPA highway mileage figures in their cars with the whole family plus luggage. It takes time to refine your techniques however.
Give yourself several months to become fully proficient and re-read the article tips often until you find yourself driving down the road constantly aware of what techniques you can be using at all times. After long enough, it becomes your natural style of driving and you can achieve excellent fuel efficiency in any vehicle you hop in to.