Originally Posted by
cooker
One factor in bike speed and efficiency is rider position. Upright seating, with handlebars higher than the seat, is slower due to poor aerodynamics and less efficient power transfer on the downstroke as your upper body weight is not leaning forward over your front foot. My rule of thumb is that ordinary commuters should have a bike where the handlebars are at approximately seat height, and you are leaning forward with your torso at about 45 degree. Bike racers, of course, and some "fitness commuters", may have the handlebars several inches lower than the seat and their upper torso leaning forward almost horizontally.
Tire pressure and texture also affect speed, so slightly narrower tires that can be pumped up to 80 psi or higher are useful, and they should have minimal tread. However, racing tires that are pumped up above 100 psi may give too rough a ride for commuting. A bike shop can help you determine what width of tires your rims can handle.
Getting good higher pressure tires is a good idea and is probably the first thing I'd do. For a given tire and tire pressure, wider tires have
less rolling resistance than narrow tires, so I wouldn't bother going super skinny.
If the OP is riding at say 12 mph or slower, aerodynamics are not going to play a huge part. If he's riding at 15 mph or faster, then being in a more streamlined position will help.
And, of course, the engine is what matters most...
Edit: If your bike does have a suspension fork, that can be robbing power as well. After tires, I'd consider swapping that puppy out...