I guess I'll disagree with the previous posts by saying the new tire size will actually be more accurate in regards to the speedometer.
With a little research, you will realize that by default, the speedometer reads fast. When in doubt, measure it with few GPS devices. You will be amazed to see 70mph is actually 67.
Stock = 215/45/17
You are proposing 205/50/17 which is a 1.8% change.
I propose you go 225/45/17 since it will be only a 1.4% change.
The 225 will also give you a smoother ride (higher sidewall), but it will have a lower risk of making your speedometer read too low.
You may of course have slightly reduced MPG figures, but in theory you should also gain more traction due to wider tires.
Last edited by edgalang; 03-03-15 at 06:19 PM.