Hey there fellas thought I would post this FYI, I found it whilst looking around the net, might come in handy for some.

It does a comparison of tyres sizes including diameters and % difference.

http://www.tyresave.co.uk/tyresize.html

Gibbo

 

Nice find, straight into the bookmarks.

 

From the Miata site, this is very similar with some illustrative work

I like the bottom text box where I can put more than 1 size and generate a table.

http://www.miata.net/garage/tirecalc.html

 

i have been using the older version for the past few years... :D

 

Interesting.. It says that you have to replace a 225/45 18 with a 255/40 18 to get the best speedo match, but would a 255 tyre even fit in the wheel arches?

 

The answer is not recommended. I know people have fit a 275 in a 8" but will you do that!?

On the other hand I am more willing to put it in a 8.5" or a 9" though..... 255/40 is such a weird size and I doubt there are many manufacturer has this size.

 

Hey if some one used size 245 x 45?

 

They'd be a bit fat ... most you'd go to is 235/40 or 245/40 ... but good luck finding 245s!

 

I have 245/40/18's on mine, and I understand that 245's are the widest you can put on an 8" wide rim (on the other hand, I was told that 225's are the narrowest you can put on a 8" rim).
Plus factors: The error is minimal, and the tyres look really great - they're so wide!
Minus factor: You have to increase tyre pressure to 250-260kPa to avoid squirming and improve turn in. After about 15-20000km, squirming does not appear to be a problem (at least in non-track situations).

 

Sorry Cromax, I missed your comment. I bought my 245/40's from my Mazda dealer, Grand Prix Mazda Aspley.

 

Well appart from the strangeness in looks as Cromax pointed out, you will be running your speedo nearly 3% to low.

Con: you are going faster than your speedo says (when you speedo says 100 your actually doing 103)
Pro: you are doing less km's than the odometer says (When your odometer says you have traveled 100km you have actually done 103).