Does anyone know how big in millimeters a 235/40/18 and a 205/65/15 are?

 

well....there are 10 mm in one cm....you do the math

 

235mm x 0.4 aspect ratio = 94mm
94mm x 2 (for top and bottom part of tire) = 188mm
18in x 25.4mm/in = 457.2 mm
total diameter = 645.2

205mm x .65 = 133.25mm
133.25mm x 2 = 266.5mm
15in x 25.4mm/in = 381mm
total diameter = 647.5mm

that's about as close as you will get any tire swap. now you're gonna ask the difference in circumference and the change in your speedo aren't you?
actually with those sizes it will be negligeable.

 

Thanks a lot. I needed that to enter into my RSM to recaulculate my mph and a bunch of other stuff.