Wheel/Tyre sizes

[Russ T Bolt]

I currently drive a Fiesta Zetec S fitted with 16inch alloys and 195/45R16 tyres.

I want to fit winter tyres on it for the next few months.

Ideally I want to buy some steel wheels for the winter, but want a different profile.

How do I calculate what size I need ?
So say I bought some 15 inch or even 14 inch steel wheels with maybe 175 or 185 tyres, what profile would they need to be ?

[Gareth350]

After a few calculations, you would be looking at
175/60X15 or
185/70X14, should give you close to the correct rolling ratio to keep your speedo correct.
Also make sure 14" wheels will clear your calipers!

Edited by Gareth350 on Thursday 19th November 16:36

[Russ T Bolt]

Gareth350 said:

Also make sure 14" wheels will clear your calipers!

Edited by Gareth350 on Thursday 19th November 16:36

Good point, will watch for that, cheers

[Nolar Dog]

Gareth350 said:

After a few calculations, you would be looking at
175/60X15 or
185/70X14, should give you close to the correct rolling ratio to keep your speedo correct.

They are miles out.

You would be better with 185/60/14's.
Popular size, throwaway prices.


They would give you a new rolling radius of 288.8mm compared with your current 290.95mm set-up.

The difference there is only -0.74% which gives you an ever so slight speedo underread.

eg: At an indicated 70 mph you would actually be travelling at 69.48mph.

Gareth's figures are between 1.75% and 5.62% over which would be circa 4mph over read at an indocated 70mph.

Get mine.

Edited by Nolar Dog on Thursday 19th November 22:20

[Gareth350]

Nolar Dog said:

Gareth350 said:

After a few calculations, you would be looking at

175/60X15 or

185/70X14, should give you close to the correct rolling ratio to keep your speedo correct.

They are miles out.



You would be better with 185/60/14's.

Popular size, throwaway prices.





They would give you a new rolling radius of 288.8mm compared with your current 290.95mm set-up.



The difference there is only -0.74% which gives you an ever so slight speedo underread.



eg: At an indicated 70 mph you would actually be travelling at 69.48mph.



Gareth's figures are between 1.75% and 5.62% over which would be circa 4mph over read at an indocated 70mph.



Get mine.

Edited by Nolar Dog on Thursday 19th November 22:20

I dont mind being put right, but just wondered where you got the 290mm current set up figure? Thats just under 12 inches & he's running 16 inch wheels with the tyre of 195 with an aspect ratio of 45, being 45% of the tyre width = 88mm+450mm (wheel size) = combined wheel tyre height of 538mm? A 185/60X14 setup would work out at 471mm. Was a tyre fitter for 15 years & we reguarly came accross these things! But it has been a long day & I could be totally wrong! Hehe!

[Gareth350]

ok, i have checked with my old work on this (they're only a couple of doors away!) on their wheel/tyre combination comparison chart,the correct sizes are,
185/55x15 &
175/65x14, so wasn't too far out!

[Nolar Dog]

Well the calculator I used gave me my figures. Bizarre!

[Russ T Bolt]

Thanks both, I will see what wheels I can get hold of