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[bob.dent]

I've been having a discussion with Albatross and Sipey (that started off in another thread) about the tyre sizes fitted to my wheels.

I have Azev 17" alloys. The fronts are 8J fitted with 235x40x17 tyres and the rears are 10J fitted with 265x40x17 tyres.

Albatross was asking why I have 40 profile tyres fited when the standard size should be 45 in order to keep the rolling radius correct.
So, the issue here is am I using incorrect tyre sizes, and should I be fitting 45's to my rims?

I have to say that my speeds seem to be reasonably accurate when moving with other traffic i.e. 50mph limit on camara sections of motorways etc.

I never thought this was an issue up until now as I'm using the same size tyres that were fitted when I bought the car. :-/

[hotel21]

Tyre sizes are a bast*rd hybrid of values....

235/45R17, for instance...

235 is millimetres and the width of the tread face.
45 is the sidewall height and is a percentage of the tyre width, ie 45% of 235mm which is 105.75mm.
17 is the rim diameter in inches.

In a nutshell, the wider the tyre the lower the sidewall percentage needs to be to ensure the same rolling radius to keep the speedo and gearing correct, if using the same 17" rims.  Will add a link in a minute or two....  

.... a good tyre size calculator towards the middle of the following page...

http://www.carbibles.com/tyre_bible_pg2.html

... plus theres LOADS of good tyre info there to keep you reading, Mr D!!  

[hotel21]

235/45/17 gives a rolling radius of 321.65 mm
235/40/17 gives 309.9 which is less than standard (but on the fronts) and may give improved handling/turn in times.
265/40/17 gives 321.9 which is only .25 mm different from standard so not much in it as far as speedo, gearing etc goes....  

[bob.dent]

So looks like I'm ok with current sizes - thanks for that Broocie.

BTW, Sipey gave me a link to this website http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC%20calculator.htm
but I was unsure how to interpret the results.

[Kevin Wood]

Might be worth going up to 45's at the front next time. Then you've got the same rolling radius front and rear and the same profile depth (given that profile is a percentage of width) so tjhey might looke better aestetically.

I'm sure the ABS would protest in the end if the wheels were mismatched but it clearly isn't enough of a mismatch in your case.

Kevin

[bob.dent]

Might be worth going up to 45's at the front next time. Then you've got the same rolling radius front and rear and the same profile depth (given that profile is a percentage of width) so tjhey might looke better aestetically.

I'm sure the ABS would protest in the end if the wheels were mismatched but it clearly isn't enough of a mismatch in your case.

Kevin

Cheers Kev.

I'll probably stick with the same tyre size then......don't really want to go confusing the ABS and I think they look aestetically ok anyway.

[Richie London]

Tyre sizes are a bast*rd hybrid of values....

235/45R17, for instance...

235 is millimetres and the width of the tread face.
45 is the sidewall height and is a percentage of the tyre width, ie 45% of 235mm which is 105.75mm.
17 is the rim diameter in inches.

In a nutshell, the wider the tyre the lower the sidewall percentage needs to be to ensure the same rolling radius to keep the speedo and gearing correct, if using the same 17" rims.  Will add a link in a minute or two....  

.... a good tyre size calculator towards the middle of the following page...

http://www.carbibles.com/tyre_bible_pg2.html

... plus theres LOADS of good tyre info there to keep you reading, Mr D!!  

this site is very good, i can understand a bit more now,  

richie

[Albatross]

Bob,

I wouldn't worry about it that much on the front, and your rears are fine.

You know what I think of your car aesthetically speaking.

Nathan