12-01-14, 11:15 PM
Checking here http://www.carbibles.com/tyre_bible_pg4.html (about 2/3rds down the page) there's a difference of 27mm in the rolling circumference. That's only 1.42% so at an indicated 70mph you'll actually be doing 69mph. What you need to know is the actual rim width. 225 width tyres should be fitted to 7.5 to 9 inch rims whereas 215 should be fitted to 7 to 8.5 inch rims. So unless you are currently on 9 inch rims, you should be OK. Have a look at one of the wheels, on alloys the size is usually cast into them somewhere.