Advice needed

[Shark]

Having read the sticky on sizes and offsets I still have a couple of questions that I hope someone can help me with before I try and put my new wheels on the car.
Front wheels:  8" x 17".  ET = 35
Rear wheels:  9" x 17".  ET = 38

Will I need spacers???

The diameter of the stock wheels on the car now, with 225/50 R16 tyres is 631mm.
This will change with the new wheels
Front: 235/45 R17 = 643mm
Rear: 255/40 R17 = 636mm OR 265/40 R17 = 644mm

Will I have any impact on speedo or odometer if I put 235 on the front and 265 boots on the back?  It\'s a 2% increase in diameter so rolling surface per revolution shoud increase by 2% shouldn\'t it?

I notice standard larger tyres are 255\'s.  Is this becuase the diameter difference is less?

Any help will be appreciated before I spend £500 on the rubber!

[scooby_si]

if i remember my old maths formula right then circumference would be piexD so be in theory would be more like 6%?
Not 100% but either way i\'ll have a play with me road angel eventually & see if there\'s a noticable difference between 17" & 18" wheels.....
Si

[scooby_si]

out of interest where did you get/work out those figures from?
Ta
Si

[scooby_si]

on this subject is the speeed taken from the front wheels i guess?
Si

[Shark]

Figures worked out on http://www.tyresave.co.uk/tyresize.html
Not sure if the front or rear wheels are used for speed/mileage so that makes for question 3...  anyone?

The wheel size seems largely irrelevant, just how it works with the sidewall percentages to change diameter.

Think you\'re right on circumference though, but I don\'t want my speedo to be out by 6% (could be the difference between 99 and 105mph LOL!) and I\'d prefer the mileage to be accurate rather than 6% low which would seem like the net result of such a change...?? :confused:

Anyone up with what the law says on speedo accuracy thresholds?  Is there a percentage variance allowed?

[Phil Bateman]

Hi Gregor,

I believe the  235/45 & 255/40 you specify are correct for 17" wheels on the FD3S but they will give a slight speedo error which is acceptable, you will be going faster than the speedo tells you.

If you want to go wider on the rear then you will have to lower the profile to keep the speedo within tolerance.

You won\'t need spacers with such a small offset which is well below standard for this car.

Regards
Phil

[Shark]

Thanks Phil.

That\'s reassured me no end.  Dunno how the spacer thing works and I\'ll be far happier just bolting them on the same way as the old ones!:)

do you know if the speedo change is related to the front or rear wheels?  just so\'s I know how much my speedo is off.

Also, do you know if the speedo is difficult (read expensive) to recalibrate?

[Phil Bateman]

Hi Gregor,

Speedo is related to rear wheels.

I don\'t know if you can re-calibrate the speedo but in days gone by we use to change the final drive ratio - not really worth it, get the right  tyres to start with.

If you go to a decent tyre/wheel place and tell them standard fit is 225/50/16 then whichever size(dia) wheel you go for they should be able to tell you the correct profile to use for you chosen width.

Phil

[Shark]

Excellent, thanks Phil.

265s on the back would have been nice, but I think the 255\'s are the way to go.
0.8% diameter change means 2.48% circumference increase.  I think I can live with that variance on the speedo.  my ability to judge between the lines on a glance at the dash probably has that amount of variance in it!!:rollin

I can live with a 2% inaccuracy ont he speedo and doubt that Mazda made any changes to the speedo/odometer setup for the models that have 235 and 255 17\'s on from the factory.

[scooby_si]

you will find all speedos are inaccurate as standard & wusually over read anyway as legally your allowed to over-read http://www.smileycentral.com/?partner=ZSzeb008\' target=\'_blank\'>http://smileys.smileycentral.com/cat/3/3_3_6.gif\' border=0> by as much as 10% but not under AT ALL, hence why they are usually slightly over anyways.....Shouldn\'t worry bout it too much just get a nice GPS device which might also happen to tell you where sum nice cameras etc are
Si

[Shark]

Didn;t I read somewhere recently that the ignorant scheming b@stards, sorry, i mean government, are planing to make such things illegal???

Am curious about the SmartNav system with PA support.  Can even book yer cinema tickets and suchlike for you while you\'re on your way!  All voice controlled so pretty cool really.  You know anything about that one?

Now all we need is such a system that can provide satnav and plan you a route that avoids speedbumps!

[Brett]

The law may grant you +- 10%, but thats also taking into account parralax errors etc.
 You wont really notice at low speeds and at 70mph that 2.xx% only equals 1.5mph.

 Of course if you go for huge wheels (why you\'d want to I dont know) then the error creeps up.  Of course the acceration goes for a sh*t too.

[scooby_si]

NB law only grants +10% no - at all!!!!http://www.smileycentral.com/?partner=ZSzeb008\' target=\'_blank\'>http://smileys.smileycentral.com/cat/3/3_2_26.gif\' border=0>
Si