Tyre size confirmation

[logicalChimp]

Guys

I\'ve got to get meself a new front tyre (inside sidewall gave out on the motorway this morning), so i\'ve been checking the current tyre sizes...

Thing is, the numbers on the current no-name tyres don\'t appear to match up.  the current numbers are:
Front: 225/35/18Z
Rear: 245/40/18Z

Now, i expected the 225/245 difference, because the rears are more \'dished\' than the fronts.  But i didn\'t expect the 35/40 difference, given that the alloys appear to have the same diameter front + back.

Is this normal, or should i get 225/40 at the front when i get the replacements?

[logicalChimp]

Update: just read the Tyre Size sticky above (http://www.mazdarotaryclub.com/forums/showthread.php?t=279)

It says i need:
Front: 225/40
Rear: 245/35

Looks like the previous owner got his numbers wrong, and put the larger diameter rubber on the rear... No wonder the car looks its snuffling for truffles

I\'ll correct the fronts next week (when i get paid) and do the rears a bit later...

[logicalChimp]

Does anyone have any comments on the above - have i got my numbers right?

I\'m getting the new rubber tomorrow - want to make sure i get something in the right size

[AtomicRex]

The second number is a percentage of the width
 
so
225/40 means the wall is 90mm.
245/35 means the wall is 85.75mm
 
They are roughly the same, but a 255/35 would be better
 
You can work out the total diameter by multiply the above number by 2 and adding (18 x 25.4)
 
So approx 640mm in diameter.  Racing tyres don\'t quote this way they are just 230/620 no need to work out the maths then!
 
Always made me wonder why they mix their units still!  Surprised the EU haven\'t put their stamp on this and insisted we move totally metric.

[Nik da Greek]

Does seem very bizzarre; diameter in inches, width in mm, sidewall height in percentage of width. Could they have picked a more complicated method!

[SiH]

Brilliant tool for you guys:
http://www.miata.net/garage/tirecalc.html
HTH

Yeah, always confused me with the imperial/metric thing - I guess old habits die hard!
Basically use the tyre calculator to pick size with rolling radius closest to stock - although i can\'t tell you what that is I\'m afraid

[AtomicRex]

Sorry didn\'t read the whole post
 
Wall thickness is a personal thing.  The thicker the wall the more compliance the tyre will have.  However then you get tyres with stronger walls than others.
 
There are many components in the car that give compliance.  You have the chassis, the suspension bushes, the suspension itself, the arb, and then the tyres.  As you make each one stiffer then the ride gets more bumpy!  And in some instances harder to get the power down.
 
Depending on the type of driving you do, and more so your style, will depict the type of tyre you go for.  It is better to have a natural rake downwards so that the front of the nose sits slightly below the horizontal giving you a form of antidive
 
So to be fair if you are going to stick with the 225/245 combination go with 35\'s on the front.  If you find the ride a bit bumpy then maybe consider upping the profile, but make sure it then doesn\'t rub on the arches.  It will roll a little more in the bends though.

[SiH]

All that is true, but one thing to bear in mind is that the car is set up for a certain rolling radius of wheel. If you vary excessively from this, then the speedo will be out, and there are reports of negative effects on handling on certain cars.
How much this affects the FD I don\'t know. But coming from a man who has a penchant and love for fitting silly wide wheels in daft offsets to cars, that\'s a bit of a moot point

[logicalChimp]

Hmm

So the first number is the width, and the second number is the side wall compliance?

Does that mean a larger second number would affect the overall diameter?  (i believe it does, but it is always good to check *before* spending money )

AtomicRex - when you talk about \'Wall thickness\' - are you talking about the thickness of the rubber from the outside through to the inner tube, or are you talking about the height of the rubber from alloy to tread?

Just trying to make sure i understand which is what, before i start talking to the tyre shop...

Edit: SiH - my speedo is already bust/giving me silly readings, and my ride isn\'t too good atm, so i\'m going to experiment.  I just need to check that i understand the terms/numbers before i go changing things.

[SiH]

There are 4 things that are the main ones to consider when buying tyres.
Tyres will read something like
245/45/17W

The \'245\' is the width of the treader area, in mm.
the \'45\' is the profile - bit confusing as it\'s a relative value - it\'s the height of the sidewall, but as a percentage of the tread width, so for the above tyre it will be 45% of 245mm - ~110mm
the \'17\' is the diameter of the wheel it\'s designed to fit, in inches.
the \'W\' is a maximum speed rating.

The overall diameter of the tyre is defined by the first three factors. What you need to do is find out stock wheel size, plug it into that tyre size calculator, decide what tread width you want, and then use the calculator to try different profiles, and see what\'s closest to stock.

That\'s a very simplified view though!

[logicalChimp]

Cheers

I\'ve got the recommended sizes for 18" wheels from the thread linked above - some kind soul has already done the maths and posted the results

Just wanted to get a 2nd opinion before i spend the money

oh - and although i\'ve currently got 245/40 at the rear, i\'ll probably be changing to 255/35 when i can afford to change the rear tyres... which should be a better match for the 225/40 at the front...

[AtomicRex]

SiH stop rewriting what I say in different words!  Its copying!!  Now go to the back of the class....and just because you threw in speed ratings doesn\'t excuse you!!
 
 
LC the wall thickness is the rubber from the alloy to the edge of the tyre...so the doughnut* bit.
 
It would be much better if they followed the racing set up and talked about overall diameters instead of percentages.
 
* this is a ring doughnutand not one filled with Jam, or anything else!

[AtomicRex]

Don\'t mix makes and models....this is an awful thing to do, especially in the wet.
 
There are many reasons for this.
 
alternative block patterns will disperse the water at different rates.
they will heat up at different rates
there will be differing grip levels due to the type of rubber used
 
All of these can lead to either understeer or oversteer

[SiH]

SiH stop rewriting what I say in different words!  Its copying!!  Now go to the back of the class....and just because you threw in speed ratings doesn\'t excuse you!!

I wasn\'t, I didn\'t see your post as i posted a new one and it scrolled to the bottom - happens a lot on the interweb

[AtomicRex]

A poor excuse!