Started to refurb my alloys today so took it down to my local garage to have the tyres stripped off. Unfortunately 2 of them have long cracks on the inside of the tyre wall and need replacing.

At the moment they are 215/35/18 BUT to keep my speedo pretty accurate should i be running 225/40/18??

Question is do i replace all the tyres to the correct running size or just 2 (so i can use the alloys)
The old tyres are pretty new and have loads of tread left.
Can i run on 2 different tyre sizes ( front and back?)

Can you also name me some good brand tyres that dont cost the earth to have. Ive seen 4x fullrun XL's on ebay for £180 delivered - ne good or crap? never had them before. same as ive seen a set of nankangs NS2 *i think* on ebay for about £200

alternatively i was thinking of a set of Falkens but they work out around £300 for a set.