Forget it, roblox dosent support realistic cars, you must always have a seat in the middle instead of on a different brick to the left.
THIS IS WHY THE OLD ROBLOX WAS BETTER, YOU NEVER HAD TO WORRY ABOUT THAT GARBAGE.
Put a customizeable seat in it and you can have the seat in the center, but your character will sit wherever you change the settings to.
NVI made this one and it works well.
http://www.roblox.com/Customizable-Seat-item?id=105226117