Assuming the wheels installed are 16x6.5" Subaru Forester wheels.
The stock tire on that wheel, from the factory, was 216/60-16
If you installed 235/60-16, when speedometer reads 60mph (96.6km/h) actual speed will be 62.2mph (100.1km/h): 3.7% faster.
The calculator says it should fit, but it doesn't tell you if the 6.5 wheel width will work with your 235 tire.
The correct size tire would be 235/50-16, which = no speedometer error.
Also...
The next time the tires are replaced on my son's '03 X, the plan is to go from 235/50-17 to 225/50-17 tires. Below is how the existing tires & wheels look.
Bobby...
[
'07 FSXT MODding Journal] [
'03 X MODding Journal]