Great and thank you for your detailed explanationOn the front 4.5J rims, the sidewalls of 175/55R15 and 185/60R15 tires will bulge out a bit but not too much. This helps to provide a comfortable ride and helps to protect the rims from kerbing damage when parallel parking at low speed.
The lower profile 185/50R15 tire is officially approved for a 5J rim. 175/55R15 and 185/60R15 tires will fit a 4.5J rim as comfortably as a 185/50R15 tire fits a 5J rim.
175/55R15 and 185/60R15 is the limit for a 4.5J rim as far as 175s and 185s go, so don't try fitting lower profile 175/50R15 and 185/55R15 tires to a 4.5J rim. Those tire sizes need at least a 5J rim.
When fitted with 15" tires, the Smart Roadster uses the same tire sizes front and rear, in conjunction with 1" narrower front rims. On a rear engined car with a light front end, the front tires' sidewalls don't need to be as rigid as the rear tires' sidewalls.
One of the tire sizes that the Roadster uses is 185/55R15. These are fitted to 5J front rims and 6J rear rims. However, 175/55R15 and 185/60R15 tires can be fitted to 0.5" narrower rims, ie. 4.5J front rims and 5.5J rear rims. The reason being that 175/55R15 is narrower than 185/55R15, and 185/60R15 is higher profile than 185/55R15.
I won’t do that either lol and thanks for the tipsThere is a cautionary tale on here of the tire place that put a front rim on the rear and a rear rim on the front, with interesting results. I don’t recall if he had the same size tires front and rear, but the car definitely had issues.
I looked at a Cadillac CTS AWD Coupe earlier this year where the same thing had been done. Fronts on rear and rears on front. Previous owner ended up replacing all the axle bearings. Not sure if that was the cause, but I wasn’t going to pay good money to find out what else might have been affected.
If you get directional tires, watch them when they are mounted. I’ve had at least 4 instances where they were mounted backwards.