SL350 2009 - 19" and two different tyre profiles

[sausage]

Hi everyone,
I picked up my SL350 a couple of months back and noted that the car was initially fitted with 17s, but someone had put MB AMG 19s on it. I then noted the tyres had different profiles.

Front 255x35xZR19

Rear 285x30xZR19

Is this normal or is it anything I need to correct next time I get tyres fitted. I am quite used to having different widths on the front and rear, but I *think* this is the first time I have seen a different profile size.

Thanks.
Mark

 

[LostKiwi]

Normal.
It's because the rolling diameter front and rear need to be the same. As the fronts are smaller they need a taller profile to equalise the diameter.

 

[sausage]

Normal.
It's because the rolling diameter front and rear need to be the same. As the fronts are smaller they need a taller profile to equalise the diameter.

Hello Lostkiwi, thanks for your reply. Can you explain a little more please?

I understand what you are saying, but that implies the following:

1) My 19" wheels are not all 19"

2) Which also implies my 19" tyres are not all 19" either.

So how do people know which 19" tyres to buy if some 19" tyres are 5mm smaller?

This sounds wrong to me. It also implies that tyre manufactures MUST ABSOLUTELY know that when they make a 19" tyre for a width of 285 that the diameter has to be 19" minus 5 mm - How on earth would the whole world be able to cope? Surely people with 19s would then end up having the wrong tyre. What happens when someone has a set of 285 width 19s that are all 19s - perhaps on a different car......some people would order a 285x30x19 and end up with a 285x30x(19"-5mm) and some would end up with 285x30x19" - it would be an impossible situation...surely ?

 

[LostKiwi]

This is nothing to do with rims.

Tyre profile is a percentage of width.
255 35 indicates the tyre sidewall is 35% of the width of the tread.
285 30 indicates the tyre sidewall is 30% of the width of the tread.

Calculating those out will show they are close:
Front comes out at 89.25mm
Rear comes out at 85.5mm
This is near enough the same overall diameter for both front and rear wheels.

The 19" refers only to rim size so measuring across the rim diameter it will always be 19".

Take a look at the willtheyfit.com website and punch in a few numbers to see what happens when you change profiles and widths.

 

[sausage]

Gotcha, thanks for clearing that up. I mistakenly was thinking the profile was a measure of units and not a percentage. It makes a lot more sense now and that is what I wanted from this post Much appreciated.