Wheel differences! Help!

[Gen5is]

Hey guys, I Just ordered 225/45/18 Kumho Ku31 from ebay.. but my spare wheel is a full sized 225/50/17.
I went on a calculator to figure out the differences in size and got this.

circumference
225/50/17 - 2032mm
225/45/18 - 2072.5mm

225/45/18 will have -1.96% speedo error

So a reading at 30mph the 225/45/18 will read at 29.41mph

Is that too much of a difference? Should i buy a 18 space saver or 18 full size and match the new specs to avoid screwing up the the diffs?

[zch91]

Your rolling diameter on your spare is out by 12.9mm, so within legal limits (normally +/- 15mm)

I can't see it being a huge issue as an emergency, definitely would not be driving on it more than you need too.

[UniqueRS]

See if you can get the circumference from the actual tyre company's, usually the actual sizes are different between brands.