Originally Posted by Mmartin
Yes, and just so you understand the numbers because I know I didn't till a few years ago the first number the 245 part is the width in millimeters the next number 40 is the ratio to the width, and then the last is the rim size. So in a 245/40-17 the sidewall is 40% of the tire width.
Yes, I knew that, but someone will learn from it. Also, I mistakenly said 245/40/17, but the actual size is 245/70/17.
For those that don't understand the reason why the 245/70 and 265/65 are the same diameter...70% of 245mm is 171.5mm, and 65% of 265mm is 172.25mm. If a 285 would fit, you would have to drop down the sidewall to a 60 series (285x.6=171) in order to keep the diameter the same.
The last helpful tip is that, in general, if you increase the actual wheel size by one inch, like going from a 17" to an 18" wheel, while keeping the width of the tire the same, you will need to drop the sidewall by 5%. So a 265/65/17 is similar in diameter to a 265/60/18, 265/55/19 or 265/50/20.