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Message: <blockquote data-quote="2slo95fiveoh" data-source="post: 279100" data-attributes="member: 8966">not to be an ass, but that is an incorrect statement.  speedometer calib. is affected by tire size on the rear, rear end gear ratio, and in some cases transmission gear ratios. for example: if your car came factory with 245/45/17... and you put 18s with 265/35/18, your speedometer would change less than 1 mph, if any. i entered my old and new tire sizes. 245/45/17 (factory 17 inch) and my new 315/30/18 into an onlinecalculator and my speedometer only reads 2 mph. higher than actual speed. On the subject of the initial thread request... certain series sidewall can differ because of the tread measurement. example:a 245/40/17 is not going to have the same sidewall height as a 275/40/17. specifics would help, as far as what tire size you are going to put on your car.</blockquote>