I have purchased the winter tire and the size is 175/65R15 front, 185/60R15 rear.
However, my friend who has 2017 fortwo said he has stability control warning issue with this size.
If I change the front one to the same size, do you think I would not get the warning issue?
If you have gone up from 165/65R15 front to 175/65R15 front and kept the same 185/60R15 rear, then you have changed the percentage difference between the outside diameters of the front and rear tires.
The outside diameter of new 165/65R15 tires is 595.5mm. The outside diameter of new 185/60R15 tires is 603mm. The outside diameter of new 175/65R15 tires is 608.5mm.
The outside diameter of 165/65R15 tires is 1.2% less than the outside diameter of 185/60R15 tires.
The outside diameter diameter of 175/65R15 tires is 0.9% more than the outside diameter of 185/60R15 tires.
So you have gone from having front tires that have an outside diameter of 1.2% less than your rear tires, to front tires that have an outside diameter of 0.9% more than your rear tires.
So the rear tires are now turning faster than the front tires, whereas before the rear tires were turning slower than the front tires. The car's ESP/traction control now thinks that the rear tires have lost grip and are spinning...hence the problem.
This is why carefully matching front and rear tire sizes is important.
You might get away with fitting rear tires that have an outside diameter of 0.9% less than the front tires, but this is a percentage based on when the tires are new. If the rear tires wear down quicker than the front tires, then this percentage difference will increase.
I would suggest that you look at changing from 175/65R15 front and 185/60R15 rear tires, to 175/65R15 front and 195/60R15 rear tires. An alternative would be to use the standard 165/65R15 front and 185/60R15 rear tire sizes.
It would be possible to use 185/60R15 front and rear, but I'm not a fan of using the same front and rear tire sizes on a rear engined car such as the Smart...much preferring to use 20mm wider rear tires than the front tires. This makes the car less likely to oversteer...which makes the car safer. Losing your rear end and spinning 360 degrees is a very dangerous life threatening situation. You don't want a situation where your rear tires lose grip before your front tires...or that dangerous 360 degrees spin is just waiting to happen.