rousseau

I don't know much about cars, I admit it. So the staggered tire sizes for the 2006 IS350 are 225/40/18 and 255/40/18, right? And the aspect ratio is 40?

I've always thought that the aspect ratio had to be the same for the front and back, but I've seen used tire sellers offering staggered wheel sizes of 225/40/18 and 255/35/18. I'm just curious: why is this possible? Could I put a set of wheels on my car with those measurements?

rousseau

Another possibly stupid question: Could I put 255/35/18 tires on my stock rims? Wherever I look the rear tires are sized at 255/40/18. Does this mean I definitely have to go with 255/40/18, and couldn't go with 255/35/18?

primavera

For an even look go with 225/40/18 and 255/35/18.
If you want bit more comfort then you can do 225/45 and 255/40.

rousseau

Thanks for the suggestion.

What I don't get, though, is wouldn't a 255/40 tire have a larger circumference than a 255/35 tire? Wouldn't that do something screwy to the speedometer and the odometer, not to mention the horizontal level of the car?

Or do 255/40 and 255/35 tires go on differently sized rims?

I guess I'm not clear on the concept. Can I put either a 255/40 or a 255/35 tire on my stock IS350 rear rims? Could I go with practically any sidewall aspect ratio?

primavera

225/40/18 and 255/40/18 is the odd combo that came stock on 2IS 250/350 RWD. Those sizes would do a significant damage to the transmission / transfer case if mounted on AWD vehicle.
It's a bit of a mystery why Lexus chose the unusual sizes on 2IS. For example on newer 3IS (2014-present) the odd size tires were replaced with 225/40/18 255/35/18.
If you need to replace your tires and want an even look then go for 225/40 255/35 and you'll be perfectly fine.
Number on the sidewall is a percentage of the width of the tire, in that case 40% of 225 mm.
Hope that helps.

https://tiresize.com/calculator/

rousseau

That's extremely helpful, thanks! And fascinating to see the speedometer error. Could that explain my speeding tickets? From now on I'll go 40 front 35 rear, definitely.

212jdmis25

the difference in the speedo reading is so minuscule from such a small change in the height of the tire, you will likely not be getting any tickets as a result.

just to answer a question you asked earlier, your wheel will accept a wide variety of 18” sized tires and not just one specific size.

rousseau

I would argue that that would go some ways to explaining it. If in Imperial the reading to actual speed ratio is 80:82.9 mph, that is a multiple of 1.03625, therefore 135 km/h, a typical maximum cruising speed that you wouldn't normally expect to be stopped by the fuzz on a divided highway in southern Ontario, would actually be 140 km/h, which is total OPP-bait and just 10 km/h shy of the "stunt-driving" threshold of 150 km/m, which is an automatic roadside impounding of your vehicle for 7 days and 30-day suspension of your license.

So that 5 km/h difference does actually mean a lot if you're not a "slow" driver.

Thanks very much, this is something I did not know. Funny, I've always though my IS350 looked just a little bit jacked up from the rear, and it turns out it really is! My next set of tires are totally going to be 225/40 and 255/35, without a doubt.

lexusDummy

iTrader: (5)

Hey Guys Need some Help and Advice on the Front Tire set up for 2006 Lexus IS350(RWD) Lowered on Tien Springs. Its gonna be a few Noob Question but need some help on this. I was told to Run Square set up better for winter(NYC) since its RWD.
What Brand Winter Tires are you guys running? Im on Bridgestone Blazz WS90
Whats the size your running Front and Rear? Mine is 235/30/18 (Front) 225/45/18(Rear Square set up) Not sure whats the Square set up for front.

Appreciate all the advice i can get since my Front just blew yesterday before getting home.

primavera

Are you getting two or four new tires?