Arh, Tyre questions. Always lots of opinions.
The original tyre size for the Z3 is 225/50/16. The previous owner likely fitted 225/55/16 tyres to try and correct the speedo error. An common practice. Going up a profile size give something like a 7% decrease in the speedo compare to actual speed and corrects the speedo error. It also clocks up mileage faster at around 7%. The mileage is correct but the speedo reads faster on the standard tyres.
The 225/50/16 tyres are not popular these days and there are fewer cheap tyre options about. Go to 205 tyres and they are popular and there are lots of cheap tyre options. I know of two Z3's running 205 tyres and they have no problems. Good ride quality. The only time you'll really notice the reduce tyre size is in the wet and during very spirited driving. You can choose your tyres for wet conditions and fix the first issue. Your driving style will depend on the second issue.
The rule of thumb for profile size is to go up a size when you go down a width. This is to keep the rolling radius about the same and keep the mileage and speedo about the same as the original tyres.
- If you where on the original 225/50/16's the equivalent would be 205/55/16.
- As you have 225/55/16 the equivalent would be 205/60/16.
- My opinion is that the more profile you have, the better the ride quality. Look at the poor ride quality and rim damage that the 20' wheels with super low profile tyres have.
There are a number of tyre spec calculators on the interweb, so you can check the rolling radius sizes and calculate the impact on your mileage/speedo.
