embrace Posted August 28, 2014 Share Posted August 28, 2014 evening all, Regards the depth of the tyre 255/45/18 etc is it absolutely imperative that you must get the same as the car manufacturer recommends? my friend has some 18" alloys and the car and tyre manufacturer is saying that he must stick to the depth of 45. He would like to buy 40's as he prefers them looking slimmer and they are about £40 each cheaper than the 45s. However there saying that if he drops to 40's his speedo would be miles out which is not advisable? I know it usually makes a little difference to the speedo changing the rim and tyre size but would swapping from a 45 down to a 40 really put the speedo miles out? cheers in advance Quote Link to comment Share on other sites More sharing options...
lolzz Posted September 7, 2014 Share Posted September 7, 2014 (edited) If changing from 255/45/18 to 255/40/18 your speedo would be reading 60 when you would be actually going 57,77mph. Its 3,7% difference in circumference, more than 3% is not advisable. (However, going from 245/45/18 which is standard for Zed, to 255/40/18, its 2,4% difference, which should theoretically be ok. 275/40/18 would be spot on - 0,1%, but then you lose slimmer sidewall.) Edit: Slimmer sidewalls might look good, but you also increase arch gap and together you don't gain much.. Edited September 7, 2014 by lolzz Quote Link to comment Share on other sites More sharing options...
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