One thing to consider is how you ride.
With skinnier/lighter tires, acceleration is much easier.
IF you tend to vary your pace, every time you accelerate back up to speed, it takes extra energy.
Going uphill also takes extra effort.
Money may be tight, but something like these relatively inexpensive tires would make life so much easier.
I replaced the 26X1.50" tires on my "grocery getter" with these 1.25" and the difference was amazing.
http://www.biketiresdirect.com/produ...et-runner-tire