Tank pack interior issue

I made this barn like interior and it has a large door so my tank(tank pack) fits into it
but when i try to drive into the gap, i end up on the roof on my interior
but when i lift my interior up so it doens't touches the grounds, i can drive with no problems underneeth it.
how can i fix this :S?

(i didnt really know where to put this so i put it just here)