2008 Passion, now a 2016
@margie You're on the right track.the tires are 15". I wanted to put same size all around. Since the tire circumference must be same as original for the ABS, I will have to put a 15" tire with a smaller circumference to fool ABS. Am looking for two now. Thanks for the input.
I just bought a 2016 Passion just like yours, and it already had the same size tires on all four corners. I often have the Traction Control System jumping in and locking up a wheel during a turn because of this very reason, and when it happens, it scares the crap out of you. Once these tires are worn down more... I'll be going back to the proper sizes, but the previous owner, like many smart owners, probably just wanted the same tires on all four corners... could be so they could rotate them more efficiently (including front to back), or could be some other personal reason.
Everyone wants bigger (usually wider) tires on their smart, but the problem is that the electronics of the car are looking for specific tire rotation speeds at specific times. When you enter into a corner (usually left handers) with any "spirit" as some people call driving fast, all the numbers come together in the computer and it thinks you are sliding, or worse yet, about to slide, so the car automatically applies braking to the opposite wheel (usually a rear) and tries to protect you from your own "spirited" driving abilities. In day-to-day driving it's not a big problem, because the math works, but then one day you'll be driving along, coasting or powering through a corner a little faster than normal, and the TCS kicks in and, as I said, scares the crap out of you. I can actually force it to happen 8 times out of 10, on a specific corner of my morning commute.
The first time this happened I didn't know what the heck was going on... the following morning on the exact same corner, it happened again and out of the corner of my eye, I saw a flicker of light on the dash... after a look into the owners manual, I figured out it was the TCS light flickering just before the system acted.
To cut a long story short, the tiny wheelbase of the smart car makes you feel like you're in a go-cart, which is fun... but when things get squirrely, the safety police inside the smart car electronics, jump in to save you. When a longer vehicle goes into a skid, you have more time (and technically distance) to correct it... but when your smart car wants to slide, it's usually already too late, and the car will spin out before you can sneeze.
There were safety requirements (search online for the "Moose Test") that had to be passed before the smart could ship into the U.S., and in order to pass these tests, Mercedes engineers put smaller wheels and tires on the front, so it would skid (understeer) before it would spin out (oversteer). In many ways, this understeer also prevents a lot of rollover accidents as well. Hence the Moose Test.
I've blabbed on for paragraphs now, just to say this... if you are going to alter your front wheel diameter, you need to make sure your rear wheel diameter is also increased or decreased in the same ratio as the TCS wants to see. There is a great tool I have found to help find tire sizes that will help you maintain proper front/rear circumference when choosing to upsize or downsize a tire or rim combination.
Good luck and drive safely... because if you don't, the smart car will try to do it for you.
Just my $.02