Thanks, the tires are due for replacement anyway so I will change to your recommended sizes. What do yo recommend for a specific tire brand for a softer ride.
In matching 165/65R15 81T and 185/60R15 84T sizes, there doesn't seem to be much choice in the North American market. My choice would be Vredestein Quatrac 5. If these aren't available I would look at Continental ProContact. The Quatrac will work well all year round including snow, the ProContact doesn't like snow and will spin and slide. I mention these two brands because you might be able to get matching tires in these two sizes, not because there are not many other good makes of tires on the market.
The size will make more difference to the ride than the brand.
Vredestein also make the Quatrac 5 in sizes 165/60R15 77H and 185/55R15 82H, although I don't know if these are sold in the North America market. It might be worth contacting the Vredestein importers/distributors or a place that sells Vredestein tires (online or physical shop) to see if these sizes are available.
Here are the sizes again:
165/65R15 81T and 185/60R15 84T
165/60R15 77H and 185/55R15 82H
The above are front and rear matched sizes as the outside diameters are similar. Don't match 165/65R15 front with 185/55R15 rear, or 165/60R15 front with 185/60R15 rear because the difference in the outside diameters of the tires is too much.
Rear tire pressures for the Smart ForTwo 451:
If you run either 185 size on the rear at the standard 36psi, you will get an unnecessarily hard ride.
For the front tire pressure you can experiment. 23psi would be enough to support the weight of the car, but 26psi or 29psi will improve fast cornering. If you don't want to experiment with front tire pressure I would suggest running 26psi on the front.
As the driver sits much nearer the rear axle than the front axle on the ForTwo, the rear tire size and rear tire pressure will have the most impact on ride comfort.
When I purchased my Smart ForTwo I found the ride unacceptably hard (the car was standard), but since I fitted softer riding tire sizes and carefully adjusted the air pressure this is no longer the case. 280 mile non-stop journeys are done without discomfort.
185s all round could be used if you have difficulty finding matching front and rear tires in different sizes. It just wouldn't be my first choice. Smart have always fitted narrower front tires to the ForTwo and they have done this for a reason. On the Smart Roadster they found it acceptable to use the same tire size all round. But that car has a much longer wheelbase, wider track and lower center of gravity. Even still, on the Roadster the owner can still run different front and rear tire sizes. For example, on the Roadster's standard 5Jx15 ET24 front rims and 6Jx15 ET-20 rear rims some possibilities include:
185/55R15 front, 205/50R15 rear
165/60R15 front, 185/55R15 rear
175/55R15 front, 195/50R15 rear
175/60R15 front, 195/55R15 rear
Back to the Smart ForTwo 451. 185/60R15 tires will fit the standard 4.5" wide front rims and 5.5" rear rims. 185/55R15 tires need at least a 5" wide rim, so won't fit the 4.5" rim. A tire fitter could refuse to fit a 185/60R15 tire to a 4.5" rim. This is because 50 series tires to 65 series tires all have the same recommended minimum rim width even though higher profile tires can be fitted to a narrower rim.
Minimum Rim Widths for 185/50, 185/55, and 185/60 tires:
Rims come in 0.5" increments, so the 4.75" as shown above is for illustration purposes to show how the minimum rim width decreases as the aspect ratio increases for any given tire width.