We did extensive research on this subject and built a total of 3 different "suckers" before we got it right.
Center to center, the perfect distance was 9" - but we used 3" wheels so the compression was 6." This may sound tight, but the rubber wheels we used were able to "suck" the tube through the narrow opening.
But the primary griping force wasn't in the wheels at all, it was in the belts that drove them. The belts were arranged in a "V" configuration, so that as the tube was drawn in the compression became even greater. The belts act like rubber bands, squeezing the tube.
It is impossible to remove an inflated tube without running the motors.
(rubber wheels not shown)