although most have explained the tyre size meaning,this is what it equates to,the tyre is 205mm wide,the 40/45 is the percentage of the width of the tyre so for example,40% of 205 is 82mm,45% is 92.25mm,it depends on how you want the car to look,the 45 profile will fill your wheel arches a bit more but if you're having it lowered then the 40 profile will be slightly smaller but the 45 profile better for pot holes and speed ramps because of more cushioning,the profile being the height of the tyre from the rim to the top of the tread.another example is I have 225/40/18's on mine which is not lowered,I don't get scrubbage,( where the tyre rubs on the wheelarch protector)unless I have a heavy passenger in the rear and then its only on high speed ramps even at slow speeds but I don't have rear seat passengers very often,so my rims are 25mm bigger,the tyre 20mm wider and the tyre wall is 90mm so 17"x 205's will fit even a GP lowered by 30mm,easily.I hope this helps,john