Vitz 1000cc OEM tyre size 155/80R13 (578mm Diameter)
Suggested tyre size by Deloy 185/70R13 (589mm Diameter)
Difference is +1.9% or 11mm which is within the OEM 2% margin of error, however i'd suggest the following:
For 14's i'd suggest the following setup:
If you want to go 15 i suggest the following:
No matter what anyone tells you, avoid 205's, they will rub on the inside of our arches ever if you use an ET 45 (offset) rim which is the limit of the vitz (ET should be between 38 & 45 to maintain propper suspension geometry)
it's best to try and stay as close to 578mm as possible. I went for 195/50R15 because it is a good balance between wider foot print (good braking & grip) & decent sidewall height (smooth ride).
Good luck on your decision & purchase.