Inpd do you have a way of measuring cadence? If you do, you can stick to a single gear for testing purposes and pedal at a certain cadence. Do not shift up or down from this gear. Do this on the same path and see what speed differences you get.
If you notice you are pedaling much faster on the Venturi vs the other bike chances are the venturi has some advantages that add up to 2mph difference. Whether it's position, mechanical tuning, or aero.

Also are the crank arms the same length?