Please check my tyre size logic

[Quietlybonkers]

I have a car with 18 inch wheels and 235/50 tyres.
The spare is missing, but I have a suitable wheel in the same pattern.
But it's a 19 inch wheel.
So I need a tyre to fit the wheel that gives me the same overall wheel and tyre diameter.

18 is inches, 235 is millimetres. One inch is 25.4 millimetres.

235 is the width of the tyre in mm.
50 is the aspect ratio, the ratio of the height of the sidewall to the width of the tyre.
So the height of the sidewall is 117.5 mm, ie 50% of 235.

A wheel and tyre diameter is one times the wheel diameter plus two times the tyre sidewall height. So the overall diameter of the wheel and tyre is 18x25.4 = 457.2, plus 117.5 x 2 = 235, so in total 692.2 mm.

If I swap to a 19 inch wheel, ie 25.4 mm more wheel diameter, then my wheel will be 482.6 mm, and I will need the tyre sidewall x 2 to be 692.2 - 482.6, ie 209.6 mm. So the sidewall height will be 104.8 mm

104.8 as a percentage of 235 is 45% to the nearest, so I need a 235/45 19 tyre.

[Grenoble]

Your user name is apt.

Won't it do funny things to the handling - different weights of wheel?

[v8will]

In theory that tyre size is correct.

In practice, If the car can carry a full size spare I'd probably want to keep to the 18"

[Grenoble]

Plus, would it fail an MOT? I can't remember exactly, but don't wheels and tyres on the same axle have to be of the same type and size (though identical brand tyres aren't needed)

[v8will]

Grenoble said:

Plus, would it fail an MOT? I can't remember exactly, but don't wheels and tyres on the same axle have to be of the same type and size (though identical brand tyres aren't needed)

A spare is temporary use only, just think how many cars have piddly little space savers.

[Grenoble]

v8will said:

A spare is temporary use only, just think how many cars have piddly little space savers.

I think you will find they are exempt, and are speed rated. I can't remember the criteria for a spare. I'm guessing they won't compare them to the main wheels in reality.

[Jakg]

Grenoble said:

Your user name is apt.

Won't it do funny things to the handling - different weights of wheel?

doogz said:

Not a great idea.

You must have the same size, width, profile, speed rating and load rating on both sides of an axle. If you're fitting a 19 on one side, you'll need one for the other.

Space savers?

Technically illegal, but a "proper" tyre (even on a different sized rim) would be a btter idea than a space saver.

Having said that, I'd look for a "proper" wheel - a kerbed up one wouldn't be that expensive?

[JM]

Use this to calculate the correct size.

As a get you home spare tyre it will probably be ok, but I wouldn't want to go too far in it myself.
Having said that I know someone who had a van with low profile tyres and new alloys who damaged one wheel and put his old spare on and ran like that for many months. As far as I know he's still alive.

How much would a 'new' wheel be from a breaker or Ebay etc?

[aka_kerrly]

With the amount of thought the OP has put in you would think that he is intending on using his spare wheel on a regular basis.

For most people the spare is rarely used and if it is, for relatively short periods. For that reason I'd simply opt for a wheel which can clear the front brakes and is close enough to the the size of the regular wheels. I'd be reluctant to lug a 19inch wheel around "just in case"

[Kozy]

On most cars it'll be fine. On some cars though an AWD system can get damaging windup with mismatched tyre profiles, and things like ABS, ESP, EBD and other wheelspeed related electronics can get confused.

Props to the OP for checking it'll work reasonably!

[anonymous-user]

Grenoble said:

Plus, would it fail an MOT?

Even if it would, you'd just leave it at home on the day of the MOT, carrying a spare isn't a legal requirement.

Obviously common sense suggests you'd want it to be roadworthy and suitable, but getting round an MOT is pretty simple when it comes to spare wheels/tyres!

[Quietlybonkers]

aka_kerrly said:

With the amount of thought the OP has put in you would think that he is intending on using his spare wheel on a regular basis.

For most people the spare is rarely used and if it is, for relatively short periods. For that reason I'd simply opt for a wheel which can clear the front brakes and is close enough to the the size of the regular wheels. I'd be reluctant to lug a 19inch wheel around "just in case"

Thanks everyone, you are all a credit to the spirit of PH.
I just want it as a spare. Just bought the car and the spare is missing. Another PHer has given me the 19 inch wheel. Kindness itself and in the spirit of PH. I won't use it any longer than absolutely necessary. And the car IS a quattro, so thanks for that piece of advice.

The maths was the easy bit - I am an accountant. Knowing that the logic was correct, and what other problems I might encounter was where I sought your advice. Many thanks all

Edited by Quietlybonkers on Wednesday 26th September 18:38