Correct tyre size

[Martin Holloway]

Hello, I’m currently driving a 2014 e 250 CDI convertible.
Just been checking tyres and a couple are getting low, then I realised I’ve got some different profiles.
255 35 18 on the back
235 40 18 on the front

I can understand the different widths but not the different profiles

anybody shed some light on the correct tyre size.

thanks
Martin

 

[Blobcat]

Hello and welcome

The different profiles are there to get the same overall diameter. 35% of 255 and 40% of 235

It would actually be better with 225 40 on the front

 

[EmilysDad]

If you drop the profile as you increase the width you keep the overall diameter the same If you had differing diameters front & rear the ABS/ESP would get its knickers in a twist

 

[Martin Holloway]

Maybe I’m being a bit thick but I can’t see how changing the width affects the diameter? Surely a 40 profile tyre on an 18” rim has a bigger diameter than a 35 on the same rim????

 

[Blobcat]

Maybe I’m being a bit thick but I can’t see how changing the width affects the diameter? Surely a 40 profile tyre on an 18” rim has a bigger diameter than a 35 on the same rim????

The profile number is a percentage of the width. So the profile needs to go up as the width goes down to get the same overall diameter

 

[Martin Holloway]

Ah that explains it, didn’t realise that number was a percentage thought it was just a direct measurement so to speak.

many thanks

I love the knowledge on this group, it’s not failed me yet!

 

[Gazwould]

It's an aspect ratio .

 

[Cole@MBS]

Keep it friendly guys! You no who I mean!! We don’t want to delete posts!

 

[davidsl500]

The OP can always use https://tiresize.com/tyre-size-calculator/ to see the difference and try the 225 instead of 235 comparison as well

 

[John Laidlaw]

Welcome Martin , late to the party as always from me but your question is answered well....

 

[JBell]

It would actually be better with 225 40 on the front

The tyre sizes are correct for the car

https://www.alloywheelsdirect.net/m...s_convertible_a207_alloys/18_amg_iv_wheel_562