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[Danny]

no but he does have 18" alloys, if they're 18x8 like yours are, you should be ok with the same size tyres as his

if anything, having 225 instead of 235 will keep your speedo as close to accurate as possible, over 2.5% either way is illegal if i'm not mistaken

bare in mind your speedo should have been calibrated in line with the OEM alloys your car had from new

if you had 16s on 225/55/16 then your 235/40/18s are only 1.33% out which is fine, but 225/40/18 is 2.55% which is too much, you might be fine putting your existing tyres on the new wheels

[MikeDundee]

no but he does have 18" alloys, if they're 18x8 like yours are, you should be ok with the same size tyres as his

if anything, having 225 instead of 235 will keep your speedo as close to accurate as possible, over 2.5% either way is illegal if i'm not mistaken

bare in mind your speedo should have been calibrated in line with the OEM alloys your car had from new

if you had 16s on 225/55/16 then your 235/40/18s are only 1.33% out which is fine, but 225/40/18 is 2.55% which is too much, you might be fine putting your existing tyres on the new wheels

Yeah just been looking at that on the carbible thingy, what a nuisance :-/

[hotel21]

Mick - not looked at the carbible link so apologies if this is a repost....

tyre numerical sizes are a hybrid of 3 different units, ie
235 - is the size/width of the treadface in mm's.
45 - is the height of the sidewall as a %age of the treadwidth, ie 45% of 235mm or 105mm roughly.
17 - is the rim diameter in inches.

Therefore if you increase the diameter of the rim from 17 to 18 inches, you need to reduce the sidewall height (take a lower %age profile) to keep the same rolling diameter of the wheel and tyre.  It gets further complicated by increasing the width of the tyre, obviously......  

235/45/17 is effectively the same size as 265/35/18......

If you check this link - http://www.miata.net/garage/tirecalc.html - it has a size calculator so you can add/exchange to your hearts content and still get the same rolling diameter and see how far out the speedo etc will be....  

[Danny]

but you want to make sure your tyres arent too wide for your arches

[MikeDundee]

Mick - not looked at the carbible link so apologies if this is a repost....

tyre numerical sizes are a hybrid of 3 different units, ie
235 - is the size/width of the treadface in mm's.
45 - is the height of the sidewall as a %age of the treadwidth, ie 45% of 235mm or 105mm roughly.
17 - is the rim diameter in inches.

Therefore if you increase the diameter of the rim from 17 to 18 inches, you need to reduce the sidewall height (take a lower %age profile) to keep the same rolling diameter of the wheel and tyre.  It gets further complicated by increasing the width of the tyre, obviously......  

235/45/17 is effectively the same size as 265/35/18......

If you check this link - http://www.miata.net/garage/tirecalc.html - it has a size calculator so you can add/exchange to your hearts content and still get the same rolling diameter and see how far out the speedo etc will be....  

Cheers for that

[MikeDundee]

but you want to make sure your tyres arent too wide for your arches

no I don't think the arches are wide enough for the tyres

[Danny]

but you want to make sure your tyres arent too wide for your arches

no I don't think the arches are wide enough for the tyres

unless you get greater offser, thus making your wkeels sit further out, giving room for a wider tyre, at least on the back, but a certain amount of wheel protrusion from the arches is also illegal i believe, and you wouldnt want wider front ones because you wouldn't be able to turn

[hotel21]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

[Danny]

always best to stick with standard

tis cheaper on insurance

[MikeDundee]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

According to Will-Floyd MBE the rims just purchased should be fine ....and if they're not i'll just blame Will, and then curse myself for a couple of weeks

[Danny]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

According to Will-Floyd MBE the rims just purchased should be fine ....and if they're not i'll just blame Will, and then curse myself for a couple of weeks

then i shall take this opportunity to point out that you bought the wheels before I said anything  

[MikeDundee]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

According to Will-Floyd MBE the rims just purchased should be fine ....and if they're not i'll just blame Will, and then curse myself for a couple of weeks

then i shall take this opportunity to point out that you bought the wheels before I said anything  

I'll ask you first the next time

[Danny]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

According to Will-Floyd MBE the rims just purchased should be fine ....and if they're not i'll just blame Will, and then curse myself for a couple of weeks

then i shall take this opportunity to point out that you bought the wheels before I said anything  

I'll ask you first the next time

and if i'm not certain i'll just say i dont know

[MikeDundee]

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts    .....

According to Will-Floyd MBE the rims just purchased should be fine ....and if they're not i'll just blame Will, and then curse myself for a couple of weeks

then i shall take this opportunity to point out that you bought the wheels before I said anything  

I'll ask you first the next time

and if i'm not certain i'll just say i dont know

I never knew in the first place :-/...and am still uncertain