So I was doing a little research for my GS400. On the decal in the side of the driver's door, the two stock sizes are listed:

225/55/16
235/45/17

But those aren't equivalent. Mathematically, the 17" stock tire/wheel setup is 1.6% smaller, which would (although very slightly) throw your speedometer off.

I guess I'm surprised because Lexus is all about precision. I guess my question is which is the 'baseline' tire size? When your speedometer shows 100mph, which tire size must you be using in order for your speedo to be 100% accurate? And, wouldn't a closer size to the 16" tire be a 245/45/17?

 

You are correct the math is off, but depending on the tire you use the math is not always correct. My GS300 stock tire size is 225/55-16 (25.7" dia.) and I will be puting 235/45-17 (25.6" dia.) tires on new rims this spring. I will be using Michelin Pilot Sport A/S and according to the Michelin web site there are close in the outside diameter. See attched file.

 

Great question. I've been told that the speedometer is calibrated depending on whether your car originally came with 16s or 17s. I find this hard to believe, but that's what I've been told.

By the way, here's a good calculator to use in determining how far off your speedo will be with various rim/tire combos (I can't take credit for this -- chuckb sent me the link):

http://www.miata.net/garage/tirecalcold.html

 

That's the website I was using! I didn't think anyone else used it but me...

Anyway, so does anyone have an idea why the 17"s aren't 245/45/17? If I'm looking to upgrade wheels/tires, should I be looking for that size instead?

 

hmmmm

My understanding was that the radius of the rubber is almost the same fro 16 or 17 inch. The rim size changes and the height of the side-wall is reduced. [You see less rubber from the side].

As the tires wear out, surely the rotations (measured) --> distance covered gets off.

Salim.

 

Well, my stock tires were 245/45/17 when I got the car brand new....

 

I think alot of people with oem 17s switch to 245/45s. Should be no prob and closer to the stock 16s outside diameter.