peteharvey

My missus has a 2014 IS250 F Sport with:
- front 18x8.0 with 225/40R18, and
- rear 18x8.5 with 255/35R18.

Can she use slighter narrower and taller profile 225/40R18 tires on the same 8.5" rear rims at the back for greater softness and quietness?
Or will this adversely affect the understeer/oversteer ratio?

Did Lexus actually modify the IS's rear suspension geometry in the F Sport model, or does it simply have wider asymmetric rear wheels and tires?
.

Answer

Technically, yes you can.
But it will look weird in the back with the 8.5 wheels. And it won't change much in term of comfort.

peteharvey

Good point. I guess it would look weird at the back on 8.5" rims, compared to the front tires on 8.0".

However using 225/40R18 x4 would allow me to switch from sports tires like the Michelin Pilot Super Sports, to much softer & quieter touring tires like the Pirelli Cinturato P7 All Season Plus.

Answer

Well, any tires brand or model can be had in about any sizes. You won't get "more tire" by using a 225/40 instead of a 255/35. They both are the exact same height.

So just get the Pirelli's in 225/40 and 255/35 and call it a day! Even if you'd put 225 in the back, you still won't be able to rotate them to the front so I see no point in doing this, maybe except a lil bit cheaper for the tires.

peteharvey

I think the Cinturato P7 touring tires don't come in 35% aspect ratios?
At 35%, I think we have to move into sports tires.

Answer

You might be right on this one, but still, pretty sure you can find a good tire in that size that isn't "ultra high performance".

berniebenz

​​​​​​​Why not?

Answer

Just noticed the OP has a 250, so I'm not positive, but on the 350, rear wheels won't fit in front due to the brake calipers being too big. Maybe they fit on the smaller 250 brakes.

Personally still I would not do it.

berniebenz

A simple trial test on either car. Will do.

minimacky

A 225/40R18 tire would be stretched on an 8.5" rim so theoretically, the taller sidewall would not equate to more comfort due to the tire stretch.

Answer

A 225/40 and a 255/35 will have the exact same sidewall height. Comfort wise, no difference.

peteharvey

A 225/40R18 versus a 255/35R18 will have virtually the exact sidewall height, however their comfort via softness and quietness will be different.
This is because the lower the profile, the more rigid the carcass must be to support the lower aspect ratio.

Furthermore, there are also different types of tire designs for different handling/roadholding and comfort characteristics.
On the one hand, there is the sporting-luxury [nee touring] tire designed for a combination of sports and comfort like the Pirelli Cinturato P7 All Season Plus, Michelin Primacy & Bridgestone Turanzas.
On the other hand, there is the more hard core sports tire designed mainly for sports eg Pirelli P Zero, Michelin Pilot Super Sports, and Bridgestone Potenza S-04.

What's more again, is that certain tire designs, models and series are limited to certain profiles.
For example, the Bridgestone Turanza T001 sporting-luxury touring tires are limited to 45% profiles.
While the Pirelli Cinturato P7 All Season Plus are limited to 40% profiles.
The touring sporting-luxury Michelin Primacies are also limited to taller profiles.

Meanwhile, the true sporting tires like the Pirelli P Zeros, Michelin Pilot Series, and the Bridgestone Potenzas run all the way down to 25% profiles etc.

Also pay attention to the load and speed ratings.
The higher the load and speed ratings, generally the less the comfort, softness and quietness...

AMNss

Get the michelin primacy mxm4 they are quiet and softer than the PSS. I tried these tyres on a rented Cadillac ats when I visited CA and they were excellent, IMO they were the best thing about that Cadillac.

arentz07

Note, the Turanzas mentioned above are not the same as the ones on the IS AWD models, those would be the Bridgestone Turanza EL 400 2.

jennypenny

Perhaps, you could try upping the sidewalls for all tyres.
225/45/18 fronts
245/40/18 rears

your speedo will be off. but you really will get more sidewall. and definitely more comfort