Width or diameter or both?

[MJ]

Hi everyone,

This is my first post.

I've looked for information on the Awd system for the IS but I can't find a clear answer.

Assuming the diameter of the wheels is almost the same, is there an issue running wider tires in the rear, like a 295?

Some have suggested a square set up, others say tire width is not an issue.

If I missed a thread on this topic please let me know.

[1990LS400]

Listen to those who recommend a "square" setup on an IS250 AWD.  I wouldn't go any wider than the standard 225/45-17.  Unless you live in the tropical part of Ontario or don't drive your IS in the winter, a 295 would completely suck on snow and the greater mass of the 295 would turn your relatively low power IS250 into a dog. 

[MJ]

I have a set of Blizzaks on their own rims for winter.

This is a fair weather set up.

The tire size I'm interested in is 255/35R18 (Assuming I don't have to roll the fenders).  

Essentially I'm dressing up my everyday driver, nothing to do with performance.

I definitely don't want to turn it into a dog.

[1990LS400]

255/35-18 should fit on the back with no problem since that was the optional rear tire size of the IS350.  The optional 18's for the front of the IS350 were 225/40-18.

The 2010 IS350 has a 306 hp engine.  The 2010 IS250 has only 204 hp.  That's a huge difference and your IS250 AWD is even heavier than an IS350 RWD.

I'll never forget my experimenting with wider tires on the LS400 I bought in 1990.  That car had a 240 hp engine.  It was heavier than your IS250 AWD but it's power to weight ratio was almost identical - I just did the math.  Wider tires turned that LS400 into a painfully sluggish vehicle.  It may have handled a little better but the "breakaway" at the limits was much more abrupt, the car no longer seemed nimble and it was less fun to drive.  And it seemed like I could feel every pebble and irregularity in the road.  My wife complained endlessly about the harsh ride.  The wider tire setup did look great however.  Fortunately, like most wider tires, they wore out quickly and I reverted to using the standard tire size. 

If you are going to do this, I suggest you use 225/40-18 on the front.  According to http://www.wheelsandcaps.com/ where I have bought extra OEM wheels, your 18" wheels should have a 45mm offset.  You won't be the first to use the optional 18" wheels and tires of the IS350 on an IS250.  Some of have been happy with the results and others have not. 

 

[MJ]

Thanks.