1985 Mercedes S-Class was available with 14" wheels and so were many other expensive cars of that era. When the Smart 450 was introduced to Europe in 1998 it was a cheap car. The first Smarts were sold with 135/70R15 front tires and 175/55R15 rear tires. However, this was a last minute change of plan for Smart after the original car failed the Elk test. Before it failed the Elk test, the Smart 450 was going to be sold with 135/70R15 tires all round or so the rumors go. Therefore 15" tires made sense. If Smart had gone with 14" rims and 135 tires they would have had to go with a high 80 series aspect ratio (ie. 135/80R14) to get a similar outside tire diameter. These would have had even worse cornering performance than the 135/70R15 tires. An important aspect to consider when selecting tires for the Smart ForTwo (or City-Coupe as it was known before 2003) is the 40/60 front/rear weight distribution. This means that you have 50% more weight being supported by the rear wheels than the front wheels on a fully loaded car (ie. 210kg passengers and 50kg of rear luggage over the rear axle). As an unladen Smart 450 weighs 730kg, this means that the fully loaded car weighs 990kg. Or almost 1000kg. And due to the 40/60 front/rear weight distribution, you get almost 400kg being supported by the front axle and almost 600kg being supported by the rear axle. I mention this unusual weight distribution because it can have a big effect on the choice of tires that you make. The original car could have had 135/70R15 70T front tires matched to 175/70R13 82T rear tires (Yes, 13" rear tires!). The load index of 70 is 335kg and the load index of 82 is 475kg. So the load indexes of these two tire sizes is almost a 40/60 front/rear split (ie. close to the weight distribution of a fully loaded car). If Smart had gone with these tire sizes, they could have gone with 29psi tire pressure all round, which would have resulted in the original car having a much softer and more comfortable ride. The tread/contact patch on a 175/70 tire is much narrower than the tread/contact patch on a 175/55 tire, so even if you inflate them both to identical tire pressures, the 175/70 tire will have a softer and more comfortable ride. Not only that, but 175/70 tires have a much higher load index than 175/55 tires (ie. 175/70R13 load index is 475kg and 175/55R15 load index is 412kg) due to their much higher sidewalls which means that you can inflate them to a lower tire pressure to support the same load/weight.
If anyone wants to try 135/70R15 70T front tires matched to 175/70R13 82T rear tires on their Smart 451, there's a company in England that can make rear 13" alloy wheels to fit the Smart (they are based on a Mini design, but the manufacturer can machine them for the 3-bolt hole arrangement of the Smart along with correct hub center bore, ie. 3 x 112mm x 57.1mm hub center bore. They don't make anything suitable for the front, so you would need to match with steel front rims.
http://minilite.co.uk/products.htm
13x5 MA1350B ETO - ET26
The product code is MA1350B and the choice of offset for this rim is between 0mm and 26mm. The Smart 451 uses a standard rear offset of 22mm, so that would be a good place to start. The 5" rim width is an ideal match for the 175/70R13 rear tire.
The steel front rims from the original Smart 450 have a specification of 3.5J x 15 ET20.5 and would be ideal for the front 135/70R15 tires which was the original tire size fitted to these rims.
There isn't much choice in matching 135/70R15 and 175/70R13 tires, although I have seen Kumho Solus KH17 summer tires in both these sizes.
If anyone did try these sizes, you would feel a lot of cushioning from the rear 13" tires but much less cushioning from the front 15" tires due to the 13" tires having much higher sidewalls, so a bit of a mismatch in terms of comfort, ie. the rear tires would be great, but you might find yourself wishing that the front tires were softer to match the fantastic ride comfort that you were getting from the rear tires. However, as you sit much closer to the rear axle than the front axle this might be acceptable.
In summary, probably best to stick to more conventional tire size arrangements especially considering the lack of grip you get from 135 front tires.