The sidewall height of a 185/65 tyre is 120.25mm, and the sidewall height of a 195/55 tyre is 107.25mm. So the 185/65 tyre's sidewall height is 12.1% more, not 10.6%.
Not only that, but let's say the height of the stiff parts of the sidewall is 25mm such as the tyre bead and the tread. This reduces the effective flexible height of both sidewalls down from 120.25mm and 107.25mm to 95.25mm and 82.25mm. So the increase of the flexible parts of the sidewall has now been increased not by 12.1% but 15.8%.
...and there's another way of looking at the difference in comfort, and that is by reference to both the tread width and the load index. However, so as not to confuse everyone I will first compare a 195/55R15 tyre with a 195/60R15 tyre.
The load index of a 195/55R15 tyre is 85 (515kg) and the load index of a 195/60R15 tyre is 88 (560kg). Now 515/560=0.92 which means that by using a 195/60R15 tyre instead of a 195/55R15 tyre you can reduce your tyre pressure by about 8% which is about 2psi to 3psi. A reduction of 2psi to 3psi in tyre pressure makes a noticeable improvement to ride comfort.
Also, if you compare a 195/55R15 tyre with a 195/60R15 tyre when not fitted to a rim, you will notice that the sidewall is actually more flexible with the 60 aspect ratio tyre compared to the 55 aspect ratio tyre...which again helps the ride comfort.
Now comparing a 185/65R15 tyre with a 195/60R15 tyre...both tyres have the same 88 (560kg) load index so use the same tyre pressure. Because both tyres use the same tyre pressure, they both have the same contact area with the road. However, the 185/65 tyre is narrower than the 195/60 tyre, which means that the contact patch with the road is narrower...but because they both have the same contact area with the road...the length of the contact patch on the 185/65 tyre is longer. This results in a softer ride.
And just like when you compare a 195/55 tyre with a 195/60 tyre when not fitted to a rim, you will find that the sidewall carcase is actually softer and more flexible with a 185/65 tyre than a 195/60 tyre...which again helps ride comfort.
There's also the affect rim width has on ride comfort. The wider the rim relative to the tyre width, the more stretched the sidewalls will be and the harder the ride. The narrower the rim relative to the tyre width, the less stretched the sidewalls will be and the softer the ride. So by fitting both a 185/65 tyre and a 195/60 tyre to a 6J rim, some of the improvement to the ride that you might expect from the 185/65 tyre over the 195/60 is somewhat negated. However, a 185/65 tyre fitted to a 6J rim will still ride softer than a 195/60 tyre fitted to 6J rim...just not as much as a 185/65 tyre fitted to a 5.5J rim compared to a 195/60 tyre fitted to a 6J rim.
The outside diameter of a 195/55R15 tyre is 595.5mm, whereas the outside diameter of a 185/65R15 tyre is 621.5mm. 621.5/595.5=4.4%
Car speedometers under-read as standard by about 5%. On standard tyres, if your speedometer is reading 70mph you are actually going about 67mph. By fitting tyres that have outside diameters that are 4.4% bigger than standard your speedometer should be completely accurate. However, it's always best to check at various speed points such as 30mph, 40mph, 50mph, 60mph, 70mph, your speedometer with GPS. Google maps now comes with GPS instantaneous speed readout, so you can use that.
195/60R15 has an outside diameter of 615mm, so will raise the gearing by 3.3% compared to a 195/55R15 tyre. For this reason, you may prefer to use a 195/60R15 tyre over a 185/65R15 tyre as it keeps the gearing closer to standard.
185/60R15 has an outside diameter of 603mm, so will raise the gearing by 1.3% compared to a 195/55R15 tyre. However, a 195/60R15 tyre will ride noticeably softer on a 6J rim than a 185/60R15...although many owners may find the 185/60R15 tyre comfortable enough...and the small 1.3% increase in gearing over the 195/55R15 tyre will hardly be noticed.
