New alloys what tyres?

[Banham]

I am getting new alloys but cant work out/understand tyre sizes.

Basically new alloys are 18x8.5 ET 48 and i want to go low on my coilovers but dont want to catch my arch. Are 225/40 or will 215/30 be better or what would be more suitable?
Also im guessing having the arches rolled would help?

[silent dubber]

i think 215/40 or 215/30 should be ok but you may need to role the front arch... all depends on how low you want to go... 

[Stevoraith]

No idea what size tyres are best for going low but you need to try and keep the same rolling radius.

If change it too much your speedo (and therfore milometer) will be out.

Try using something like this calculator;
http://www.alloywheels.com/Tyre_Calculator

Going from the standard 225/40/18, you could get away with 215/40/18s but that's about it.

It helps if you understand the tyre sizes- the first bit is the width of the tyre in mm.  The second bit is the sidewall height- it's expressed as a percentage of the width of the tyre.
So 225/40/18 is 225mm accross the tread, the sidewall is 40% of 225mm (90mm) and the rim diameter is 18inches.

If you are sticking with the same size wheel, you need to try and keep the sidewall to roughly the same height (90mm in the above case) to avoid making your speedo read differently.

[Banham]

No idea what size tyres are best for going low but you need to try and keep the same rolling radius.

If change it too much your speedo (and therfore milometer) will be out.

Try using something like this calculator;
http://www.alloywheels.com/Tyre_Calculator

Going from the standard 225/40/18, you could get away with 215/40/18s but that's about it.

It helps if you understand the tyre sizes- the first bit is the width of the tyre in mm.  The second bit is the sidewall height- it's expressed as a percentage of the width of the tyre.
So 225/40/18 is 225mm accross the tread, the sidewall is 40% of 225mm (90mm) and the rim diameter is 18inches.

If you are sticking with the same size wheel, you need to try and keep the sidewall to roughly the same height (90mm in the above case) to avoid making your speedo read differently.

Thanks for the help guys
Tried searching for 215/40/18 tyres and not many came up/unheard of tyres 

 I think i may stay with 225/40/18 and just get the arches rolled to give that little bit of extra room as they are et48 but would really the look of 215/40