I usually ride a road bike and even if I sit completely upright, I notice I immediately lose about 1-2 mph in speed. When in the aero bars I gain 1-3 mph in speed for the same effort on the hoods. Beyond aerodynamics, tire pressure and style has a huge effect on speed. My 80psi MTB slicks gave me an instant 3-4 mph gain over my lower pressure knobbie on my MTB. But beyond that, my switch from my MTB with 80 psi slicks to my road bike with 23c 120 psi tires, in the same relative body position resulted in not much increase in speed.