Safety. Narrow front tires have a tendency to induce understeer during sharp maneuvers at speed. Automakers and safety experts believe that it is easier to recover from an emergency where the front end plows forward, as opposed to oversteer, where the rear end kicks out.
So the story goes, back in 1998 it was discovered that the original 450s could roll during an emergency maneuver. The Mercedes-Benz A-Class, a car developed alongside the 450, experienced a rollover during the infamous "moose test." Smart's engineers fixed this by lowering the vehicle's center of gravity, widening the track, changing the steering, and stiffening the suspension. Further, stability control and the narrow tires keeps things on the ground and controllable.
The 451 is wider and longer than the 450 (and the 453 wider than the 451) so the risk of rollover is low in the newer cars. But keeping the staggered setup still ensures that in an emergency, you get predictable understeer.
THAT sounds like the absolute truth to me. If anyone knows it would be YOU or another long timer! Plus jwight gave his blessing so I will take it as gospel. Thank you MM! My 2012 has four 6" wheels so I assumed that someone wanted to rotate the tires so they put 6" wheels on the front to enable that. I'm not sure how important rotating tires are on such a light car? However, I know for a fact that oversteer is dangerous and could be deadly. My 2011 451 has the correct wheels in the correct place. Fortunately they both needed New sneakers and since they both have Brabus wheels the 175's presently on the front of the 2012 won't go to waste. I WILL need a pair of 15"x 5" Brabus wheels to make it right again though, any ideas?
I put 17mmf spacers from Smart Madness in the back with 205\55-15 Kumhos on the back and 185\55-15 on the front. I think I'll go back to the Special Motorcycle Only tires again in 165\55-15. (Kenda Kanine II) The Over steer with the 185 makes it drive like a Winston Cup car. The 165 with no spacers gives it a noticeable understeer. I'll post after the transition. For the doubters, I got 14,900 miles out of the fronts (2000# car vs 700# bike?)
Since 2014, my Smart 451 Turbo has 7.5x16 rims with 195/45 tires all around. On dry roads, the handling is much better because there is no more severe understeer, instead the Smart behaves only slightly understeering to neutral.
But if you are too fast on a wet road with a narrowing corner, it can happen that the rear end becomes light when braking - it is then enough to open the steering a little to prevent oversteer. Normally, the Smart would push over the front wheels in this case. However, I never drive so fast on wet roads that it becomes a problem.