Tyre sizes on 91 rims

Janet & Jeff · Post by **Janet & Jeff** » Fri Apr 02, 2004 9:35 pm

I would like to fit some larger tyres to the original NA rims, is there any
problems with 195/50,s going onto the original 7 spokes
I.E for warrenting purpose,s would it degrade the handling, steering ?? I
realise that the speedo readout will be slightly different to actual speed and
hope one the engineering boffins could advise how much,
Thanks guys.

Fletch · Post by **Fletch** » Fri Apr 02, 2004 9:58 pm

Hi there...

You forgot to mention the critical dimension, which is the diameter of
the wheels you already have.. I suspect it is 14 inches, if you have
the stock 7 spoke alloys...
But some years came with 15"

The stock size for 14" is 185/60-14.
For this wheel, 195/55-14 is the correct upgrade, and 195/60-14 will
also work. 195/50-14 will probably fit, but its a bit small. 205/50-14
is better, but not a very common size.

If you have 15" wheels, "stock" is 195/50-15 and a common upgrade is
205/50-15.

Basically, you want to keep the rolling circumference/diameter within 2%
or 3% of stock.

185/195/205 is the width of the tread in mm. 50/55/60 is the height of
the side-wall, as a percentage of the tread-width. 13/14/15/16 is
diameter of the wheel, in inches.

So, remembering that the rolling diameter of a tyre is the diameter of
the wheel, added to two times the height of the tyre side-wall... you
can figure out which tyre/wheel combinations are within 2/3% of the
stock size. (remembering that 1 inch =3D 2.54mm)

The 2 or 3% is not a hard number, but outside that will start to affect
acceleration and speedo-calibration to a significant amount .

Fletch.

Grant · Post by **Grant** » Fri Apr 02, 2004 11:41 pm

Try this site for tyre size calculations.
http://www.tyresave.co.uk/tyresize.html or
http://www.miata.net/garage/tirecalc.html
Just put in the standard size then the size you want to fit. It will show
you the percentage difference in diameter so you can work out the speedo
error. If you can borrow a GPS, then you can accurately measure the actual
speed of the car at present and compare the speedo reading. You might find
that you will finish up with a more accurate reading!
195/50's will be 4.7% too small.
Grant.

jeff · Post by **jeff** » Sat Apr 03, 2004 12:04 am

if he means 195/60 x 14 then it's about ~5% bigger
diameter... happily the speedo is usly about 5%
optimistic. 2 wrongs *do* make a right!!

That was my take on the situation from some time back
anyhow

--- The Simpsons <grant@simtronics.co.nz> wrote:
[...]

Lou Girardin · Post by **Lou Girardin** » Sat Apr 03, 2004 7:26 am

Your car will accelerate slower, lose top speed and ride worse, but may be
imperceptably faster in a corner.
Sounds good to me.
Lou

Scott · Post by **Scott** » Sat Apr 03, 2004 12:09 pm

And besides if your wheel/mags differ by more than 5% of the original
wheels total diameter then you have to get it certified. So says the
cert page in the latest performance car magazine.

Lou Girardin · Post by **Lou Girardin** » Sun Apr 04, 2004 5:01 pm

Anything bigger than 185 on a stock 5.5" rim will give you too much sidewall
flex.
Lou

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