I posted this question on the RDX board, but there doesn’t seem to be much traffic. Hoping someone here has knowledge of the RDX, or just tires in general.

I had taken a set of Continental tires off my wife’s ‘09 TL SH-AWD before she her accident and wanted to give them to a family member who needs tires for her ‘10 RDX. The continental tires are 245/45ZR18 and the RDX tire size is 235/55/18. Would the Continentals fit on the RDX?

Thanks you! 8

 

No, the second number changes the sidewall height and makes them the wrong size on the outside. The first number is the only one you can mess with a little without changing the rim size or jacking up/lowering the vehicle, unless you are doing a plus one/minus one... but that's beyond the scope of your question it sounds like.

TireRack.com has some good resources for messing around with tire sizes on a car that may help you understand it more.

 

I'm no expert, but I think badbart's answer is incomplete. By changing the "first" number, one automatically affects the "second" number, as that second number (sidewall height) is simply a ratio relative to the first (tread width). To change it at all, one would need to change the first and second together. For example, for going to a winter tire, some guys like a narrower tire, so they decrease the tread width (first number) and increase the second (sidewall); if done correctly (this is where those resources he referred you to come in handy) this would preserve the accuracy of your speedometer and odometer. Going from a 235/55 to a 245 width, I think you'd want a 50 sidewall. It's not a perfect match, and would likely have some effect on the speedo, but nowhere near as much as going to a 45. You'll also need to be aware of whether you have clearance for a wider tire, though in this case, the extra 1 cm shouldn't create an issue.

 

I am not a tire expert, so I am sure I oversimplified it. Sockpuppet is not only too sexy but probably "more correct". There are ratios involved. However the 55/18 and 45/18 part are to me going to indicate a decent outer diameter difference of about an inch I believe without looking up the math, regardless of the 235 or 245 width equation.

 

About an inch and a half decrease in outer diameter. Not sure how much that would throw the speedo off, but it would be too much for me, I think. My earlier comment should have noted that even if one "corrects" by tweaking the sidewall ratio, there are limits on what tire will fit what rim. Again, use the resources badbart mentioned, or even just talk to a tire shop.

 

Here ya go, plug in the numbers and compare 2 tire sizes:

https://tiresize.com/calculator/