It maybe can take it for the Speedo tolerance, but the ride will be much harsher with 11mm less sidewall. Use a sat nav to see what speed the Speedo in the car is doing at 70mph when u get the chance. My car currently read 72mph on the Speedo for the GPS to read 70mph. So its 2mph off. The calculator says my Speedo should read 70 and actual speed is 69. Well I know at 70 its actually 68 on the GPS. So for me going to bigger size of 205/60/15, the calculator says +3% (+3mph) therefore my Speedo at 70mph will be 73mph actual. Under normal circumstance this is a little too much, but I already know at my current size it's already -1mph. So it should read 72mph actual at Speedo 70mph and that is within the tolerance.
You can if you want it just means you will have a lower profile tyre that's all
U want to know if your car came with that profile originally, this is important for changing tyre size for Speedo tolerance. Tyre size calculate Here too If your car did came with it originally then going 225/40/18 isn't viable.
Can I use 225/40/18 for 225/45/18 as I didn't know I could use different sizes. As 225/40/18 is lot cheaper for my bmw. Thanks in advance.
While mid range tyres may not perform to the standard of the best, they are reasonably good enough for most. They are considerably cheaper too of course, the likes of falken/uniroyal and others. So if money was no object then get the best.