Mazda Rotary Club

I have a set of 8x18" wheels and would like to know what is the best size tyre to go for.

Would the handling be better if I stuck with 225/40/18\'s all round or am I better off going for wider on the back.  Say 245/35/18 rears.

I\'ve been trawling for ages and can\'t seem to come up with any decisive answers.

Thanks guys.

PS.  Bill Shurvinton always posts excellent technically correct statements, so please be my guest to comment here!

Mark.

depends on offset on the wheels i run 235/40/18 on front and 265/35/18 on rear that is 8x18f and 9x18r

I\'m no expert but I\'m pretty sure that I read somewhere that you shouldn;t put wider than 235 on 8" wheels.  With the low profile of 18\'s this would bulge.
I don\'t think 245\'s would be a good idea  but try out the following websites as I\'m sure I read sommat on one of them.

http://www.tyresave.co.uk/tyresize.html
http://toy4two.home.mindspring.com/offset.html
http://www.yokohamatire.com/utrimwidth.asp

hope this helps.

Thanks Shark,

I have just been looking at those very sites!

I found this one:

http://www.chris-longhurst.com/carbibles/index.html?menu.html&tyre_bible.html

If you scroll halfway down, it gives you a calculator for selecting rim and tyre widths.  It reckons 8" rims will house 245\'s ok but I\'m still not sure.  I suppose I need to look at the offset as RX7Veilside said above too.

Thanks guys, still not sure!

Mark.

yeah, defo check the offset, but I\'d try and get advice specific to 18" rims.  not a lot of sidewall height there to soak up the extra width...

OK,

The offset of the 18\'s is +38mm which according to one of the above calculators will extend the outside edge of my wheel by 18mm.  I don\'t see this as a problem as there is plenty of room in there for them to fit.  Back to what width is best.
225 all round or fatter on the back....  What do you reckon??

Mark.

+38 would fit 235\'s on the front with no rubbing at full lock  I run 235\'s on an offset of +35 and that\'s quite tight at full lock.

bigger at the back will give more traction in a straight line but more like ski\'s if you hit standing water or ice patches :o
- really depends what power you\'re running as to whether or not it\'ll make any difference.

I run 255\'s at the back like many and the ride seems a lot less harsh than when I had 225\'s all round on stock wheels. (oh yeah, and I run 17x8 on the front and 17x9 with et 38 at the back.

G

Thanks again Shark, I have a single turbo going in soon so I\'m upping the power a bit.

I am fancying maybe 225/40/18 on the front and 245/35/18 on the back....That is unless someone advises against this.  From what I have read in the last 5 hours(!!), I think this should be ok.

Thanks again for your help,

Mark.

Soz for the hi-jack Mark...

Gregor you run 235 and 255 on 17\'s, what profile??

Mine currently has 215 and 235 (both 45 profile) but I have just ordered 225/45/17\'s for the front (17x8) and I will be getting 255/40/17 for the rears (17x9) what profiles do you run??

These are only 0.2% different in rolling radius to each other.

Cheers

Mark, that page you found was great :)

I had worked out the tyres I thought I would need and it\'s nice to have that confirmed with those cool calculators he has :)

One to be bookmarked!

I\'m afraid I can\'t help you in the wheel department, as I\'m moving to extra wide and bigger rims: -

FROM
Front 8.0J x 17" with 235x45
Rear 9.0J x 17" with 255x40

TO
Front 9.0J x 18 with 255x35
Rear 10.J x 18 with 265x35

A lot of this depends what feel you want.

Im going to run 235/40/17\'s all round on 9-10 inch rims (depending which I use at the time).  Should be good, with a nice sharp turn in and not a lot of wobble.

Would use 225s on the front, but I have loads of 235\'s so they will have to do.

Wiggy,

I run
Front 8.0J x 17" with 235x45
Rear 9.0J x 17" with 255x40

The backs are the ones linked to speedo and odometer so you really need to keep similar circumference to the original stock wheels (225x50 R16) or alter the speedo somehow.

I don;t think the difference in radius between front and back makes that much difference really

my 255 17\'s mean my speedo reads very slightly over but only by about 2.5%

an extra 2.5% you mean they always over read by upto 10% as that\'s wot yer legally allowed if yer worried just get sum1 with a road angel or GPS stuff to check how much you over read always nice to know how fast yer actually doing should plod need to stop you ;)
Si (more speedos than he knows what to do with....:D)

I used to have 245\'s on my rears and they were 8 inch and had no probs. Hope that helps.

Thanks Jetpilot, that was the response I was hoping for from someone!  Nice one.

Mark.

Gregor

the difference between front and back can screw up your ABS system if the difference is too much.

Std size wheel of 225/50/16 = 1983.60mm Calculated Rolling Circumference.

I have chosen 225/45/17 fronts = 1992.71mm (0.5% bigger)
I have chosen 255/40/17 rears  = 1997.42mm (0.7% bigger)

Giving only a 0.2% difference front to back

Both of these are within the tolerance allowed for ABS system to function correctly.

Also because of the slight difference in size my speedo will read slightly slow. going off the rears this is what I get.

Revolutions per Kilometre   old 504.1     new500.6
Revolutions per Mile                   old 811.3   new805.7

Fronts

Revolutions per Kilometre   old 504.1     new 501.8
Revolutions per Mile                   old 811.3     new 807.6

regards

hadn;t thought about ABS bud.

Surely the 255 235 combo is okay though as Mazda used it on some of the cars?

235/45/17 are 1.2% bigger than 255/40/17 and will be fine as far as ABS goes.

I think your allowed about 4% difference before it might srew up the ABS sensors. You can go to a 45 profile on the rears and still get away with it at 2.7% difference.

Cheers