Why to Keep Those Tires Pumped

Lets start off with the all the stats for the week, I'' just recite what my odometer says before I reset it ;)
Distance Traveled: 123.67km
Average Speed: 21.3km/hr
Maximum Speed: 54.0km
Calories Burned: 1986.5
Odometer Total: 1181.1km
Days I didn't bike: none

Now onto the tires.
I'm coming at things from a 'knowing very little and learning as I go' perspective, so if I say anything oober obvious just roll with it ;)
Over the past week I seemed to be getting less and less out of my battery, normally I get ~15-20km per bar of my battery, but I was getting less than 9km per bar. Something was up so I decided to get to the bottom of it, this meant going to my 'go to guy' a.k.a-Dad, and stating my problem then waiting for a calculated solution.
Next morning Dad said he had pumped my tires up, 'they weren't low or anything, just not full,' was the comment I got from Dad. (It's at this point that I would like to point out that I do now how to pump a tire up, my Dad had just been awesome and done it after I went to bed, thanks Daddy xx).
I went on my merry way to college and when I got back that night (a round trip of just over 15km) I hadn't even gone through a bar of battery yet! Before tire pumping I was going through two!
Now for the nut-shell explanation about why keeping your tires pumped uses less battery (it's at this point that you can please feel free to comment with better explanations if you like :P ). Basically when your tire is flat there is more of it touching the ground, resulting in what some might call 'tire drag,' This is were the excess tire that is touching the ground is actually causing friction (resistance) and therefore you need to work harder to move it. Physics wise the more surface area contact an object has with the ground the more energy must be expelled to move it and combat the friction. The same goes with your battery, not only are you having to work harder to keep the tire rotating but so is your battery (hence you get a lot less out of it).
At the end of all this I will now be checking my air pressure once a week and topping up as needed, it was a very gradual change so I didn't notice at first but when I compared my 'normal' battery usage with what I was getting a the end they difference was huge! I was needing over 50% more battery with flat tires, I sure wont let that happen again.