Tyre size conundrum

[LTD]

Currently have 265/40 on 18*9 rims fitted to the rear of my S500

Needing to change tyres but options are very limited at this size.

On that basis I could go up or down a width. Dropping or raising a profile size doesn't fit the maths as far as I can see.

So, do I look at 255/40 or 275/40 ?

 

[flango]

Plenty of options on this size you got anything in mind? Our supplier can get most brands some not even available in UK only Germany. We have been banging winter tyres out for the last 2 weeks whilst everyone is complaining there is no availability

Let me know what you fancy and I can get them priced up for you and even bring them up next time I'm in Paisley if anything suitable.

 

[Bellow]

If you need or want to retain the overall rolling diameter then for the same diameter wheel the section width multiplied by the aspect ratio have to equate.
eg. 265 x 0.4 = 106.

So for a 255 you need larger than 40, 275, less.

 

[LTD]

If you need or want to retain the overall rolling diameter then for the same diameter wheel the section width multiplied by the aspect ratio have to equate.
eg. 265 x 0.4 = 106.

So for a 255 you need larger than 40, 275, less.

255 * 0.40=102
255 * 0.45=114.75

So, 255/40 is closest to 265/40

275 * 0.35=96.25
275 * 0.40=110

So, 275/40 is closest to 265/40

 

[Bellow]

Or 245 x 0.45 = 110.25
285 x 0.35 = 99.75
Shows that a 20mm variation in width is required to change aspect ratio by 5% at these dimensions.

Moving + or - 10 mm while retaining 40% is probably within tolerance for the electronics, giving you the option to alter width and consequently gearing slightly in whichever direction suits you best.

 

[flango]

Don't forget the insurance implications, changing to a tyre size other than that specified will be classed as a modification also reducing the tyre width on a car like this even by 20mm is a seriously a bad idea.

 

[Bellow]

Nor would I recommend a 20mm reduction in width (not 10mm even), but was just crunching the numbers to show how the correlation of width and profile affect diameter.
Apologies if I caused confusion.

 

[raymont]

265 40 18 has a circumference of 2102mm

255 40 18 has a circumference of 2077mm

275 40 18 has a circumference of 2127mm.

This is a difference from 265 size of 1.2 percent, up in one case , down in the other. Insignificant.

Changing the aspect ratio will cause a bigger change. E.g. 255 45 18 has a circumference of 2157mm which is a change of about 2.6%.

 

[jaymanek]

I would go to 275, nothing sillier than skinnier looking tyres on the rear of a car compared to the front! The 10mm really is insignificant...

 

[flango]

I would go to 275, nothing sillier than skinnier looking tyres on the rear of a car compared to the front! The 10mm really is insignificant...

I agree if you are going bigger but it's very significant if you going smaller, particularly if you drive the car in a spirited manner

 

[LTD]

So I really ought to be looking at 265/40/18 or 275/40/18 then .......

 

[rigel]

excuse my ignorance here, but does changing the outside tyre diameter not affect the reading the speedo displays?
went to look at an e220 with low profile tyres fitted and the seller didn'k know himself.

 

[Chris Murray]

So I really ought to be looking at 265/40/18 or 275/40/18 then .......

2754018 is the approved size for the 18x9.5 AMG I wheels on rear axle. You'll be fine with that size on 18x9.

People do love to get their calculators out don't they?! Put them away!