Tyre sizes!

[11293]

I have a 1990 560SEL. I recently purchased a set of 17" alloys from Eden Park Motorist Centre.
They put on a set of 225/45 Michelin tyres, which they said are the right size. However, on driving home I noticed the speedometer was clearly not registering the correct speed. It felt about 10mph out at any given speed. I did some investigation on the internet to try to find out the correct tyre size. There are couple of helpful sites, one of which is 'Tyresave calculator'
What I am confused about is the size of the 'profile'. The calculator suggests tyres at 235/45, the critical factor being the overall diameter of the tyre, which should be as near as possible to the 26" of the original tyres on 15" rims. How can a 45 profile be different for tyres of different widths? Does anyone know what the correct tyre size should be for 17" rims on my car? Eden Park were helpful in saying they will change the tyres but say the profile remains the same for 225/45 and 235/45. They offered to put a set of 225/50's on but strangely the tyresave calculator suggests that size will not be significantly different from the 225/45's which were put on. Can anyone throw any light on this without getting too technical - I was never good at maths!!

Howard Fisher

 

[paulcallender]

The profile, is the ratio of the height to the width. It relates to the width of the tyre, not the wheel. So it follows, that a 235/45 tyre is slightly bigger diameter than a 225/45 tyre.

Remember these facts, assuming you ensure rolling diameter remains roughly the same:

Larger width tyre (and possibly wheel): risk of rubbing bodywork; greater rolling resistance; greater track (which will increase wear on suspension components but provide better stability on corners); better ultimate grip in the dry; worse grip in the wet; worse ride (will hit a greater proportion of bumps!); more expensive tyres.

Larger diameter wheel: necessitates lower profile, therefore reducing ride quality for a given width; more expensive tyres.

To summarise, the only technical benefit of larger wheels/tyres, would be ultimate grip in the dry and extra stability. Everything else is a negative. Of course, I've missed out the subjective issue of looks....

 

[guydewdney]

the 45 you mention is a RATIO of the width - its a percentage..

so a 100 wide tyre with a 100 section is 100 high(100% of 100)

a 100 wide tyre with a 50 section is 50 high...(50% of 100)

a 200 wide tyre with a 50 section is 100 high (50% of 200)

so a 235 tyre with a 45 section is 105.75 high (45% of 235)

a 225 tyre at 45 sectionb is 101.25 high (45% of 225)

now - the triky bit - this is in millimeters - and the rim is in inches...17" = 431.8mm

total diameter is 431.8+101.25+101.25 = 634mm (ish) (wheel + 2 x tyre wall height)

add into this Pi and stuff - and you find that a small difference makes a big difference to 'rolling circumference' - whch in turn screws up the speedo... thats why they made a tyre calculator...

 

[paulcallender]

In the defence of larger (diameter) wheels, I forgot one advantage: they allow you to use larger brakes. This is more of a consideration for the guy designing the car, not really a cost-effective (or necessary) upgrade.

 

[pascal]

That is why the malest wheels you can fit on the later R129s are 17" for the 330mm front disc.

I think when you use the term profile, you mean sidewall width.

As for 10mph diff. I make it out to be 1.5%. Having said that I would think that your rolling circumference should be about 2040 to 2050

The 225/45 r17 gives 1993mm
The 235/45 r17 gives 2021mm
A 245/45 r17 gives 2049mm (as on my car as standard)
A 215/50 r17 gives 2032mm

Depends on the width of your rim which choice you have.

If you give rim width of new wheels, plus your old wheel/tyre sizes, we can make a near perfect match

 

[11293]

At last I begin to see the light! Many thanks to you all for your help. I have leant more about tyre sizes in the last week than I have ever done before. I will now go back to the company who I thought were wheel and tyre experts!! Seems I now know more than they do!

Howard

 

[mlc]

Just one thing to add to all the expert words given.

You state that "It felt about 10mph out at any given speed", if you have the wrong wheel size for your car you will be getting a percentage over or under estimate error. I cant see how it could be just adding or losing 10 mph all the time.

Perhaps the lesson to by learnt here is that lots of so called eperts are really just cowboys ready to take your hard earned money.

Mark.

 

[pascal]

OOps yea Mark,

Howard said 'at any given speed' not at 'a given speed' as I thought.

I think it may be a faulty gauge. In any case, it worked out at only 1.5 %