My mother owns an 06 Saturn Vue Redline (go figure, old lady in a sport model) and her tires are spent. Stock size is 245/50/18. This size is pricey no matter where you look. If I bump the aspect ratio up to 55 I can get her some nice rubber at almost half the cost. How much does the sidewall height increase based on bumping up from 50 to 55? Speed rating does not matter.
245/50/18. 25.4mm=1 inch…therefore your width of 245mm=9.8" The second number, 50, is the aspect ratio used to give sidewall height…in this case it is 50% of the width of the tire…so take 9.8" x .5 to get sidewall height of 4.9". Sidewall+wheel diameter+sidewall= tire height. 4.9+18+4.9=27.8"
245/55/18 sidewall height=5.39"
Looks like 225/55/18 would get you an almost perfect match diameter wise. @60mph indicated she would actually be going 60.2mph. Less tread width obviously but it doesn’t sound like she’s really looking for max performance. The narrower tire would be better in the snow as well. That seems to be a pretty popular size as well since discount tire has lots of options in that size.
Just be warned that some tire shops treat that OEM tire speed rating like the word of LS2 Jesus himself and may give her a hard time about going to a lower speed rated tire. Another great reason to buy online and have someone like Paul @ Swerve mount them.
They make formulas for this, just use google to find it and you will know how to figure out any adjustment you ever need to make in the future.