I'm certain I saw a table on Roger Ford's site about the MK4 Vs MK3 and the tapered sides making the MK4 more aerodynamic. Not much though. It only became significant over the full life if the vehicle.3 members of the 15X class have tapered bodyshell's as do PEP's yet aren't ever going to tilt. I think it allows for better route availability but don't quote me.