Hi.I am changing the rims on my 2014 IS250 AWD.Currently they have 255/35/18 tires on the back but would like to change the width to 225 or 235 because the wheels I found are 18x7.5.I dont plan to change anything else but the width.Will this cause any problems.I appreciate the help.

 

I changed mine from 235 to 265 on my AWD RX.
No issues, but more resistance = poorer MPG.

 

You would need to make sure the tire diameter is overall as close as possible to its original diameter. If not, there could be damage to the AWD system.

I like to use https://tiresize.com/calculator/ to make sure the sizes work out. As far as adjusting the width, however, I don't think it matters, other than the width's effect on the overall diameter.\

So for your case, a 225/40R18 would be ideal.


The thread author wants to change from a 255 to a 235. Not sure what you mean. The RX is a different vehicle anyway.

 

If you keep the OEM 225/40-18 up front and go with the same size in the rear, everything will be ok.
For an AWD vehicule, it's important that the rolling diameter be the same on all four corners.

 

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Just a note, to keep the rolling diameter of any two tires the same, you can adjust them by keeping the air pressure adjusted so the radius (or measurement) of the ground to any part of the wheel is the same. So, let's say you measure from the ground to the center of the wheel, if it's the same, you have the same rolling diameter regardless of the stated size of the tire. This is not to say that each tire would wear with the same pattern on the tread surface.

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This feels like a "technically..." kind of solution, but also seems like it's not the best idea for long term wear and reliability.

Better to just have the same rolling diameter when the tires are at similar pressures.

 

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What you say is true, however, it's a fallacy to presume all tires of the same manufacturer's stated size is the same. Each tire in a batch or model is slightly different. If one tire in a pair on one's car, or bunch, is used for just a short distance, the diameter is different. If the pressure is just a couple of pounds different, the size will be different. If the weight on one side is different, the size will change. Sometimes the amount is slight, but after some miles will be accelerated.

My point is that no matter what your tire markings say, the only accurate way to assure that you have the same rolling diameter on two wheels is to "technologically" measure as I stated. Most of the time, for an open differencial, it will not matter if there is a slight difference. Front ends of most cars are "open" totally all the time, or for a 4WD may have a limited slip which is half open. Which is why, suppose you used a new tire for, say 5000 miles and needed to replace it as the opposite tire got damaged, most folks would not discard the other. I explained how you can compensate most of the time for a slight difference.

Usually we just take the easy way and make all pressures the same. As I said, that gives you a ballpark range that will work. When you will really notice is if you had a Detroit Locker in the rear which reacts to different diameters by pulling more on the larger one and will accelerate the wear on it. Fortunately most of our cars have an open diff which will allow for the difference.

That's my opinion and I'm sticking to it!

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If what you're saying is to try to compensate for more tread on a newer tire versus an older tire, sure, I guess that is valid. But, as you stated, it can cause tread wear issues over time.

And the minute differences in manufacturing or variance across different tire brands/models for a given tire size is not likely to cause an issue. If they did, we'd have issues with differentials all over the country.

I just don't want to confuse. The best solution here is to replace the tires all at once - though that isn't always practical, if the difference in tread depth is relatively small front-to-back, it's gonna be okay. An anecdote - I replaced one tire on my IS 300 AWD when the other tires on the car had roughly 6-7/32" remaining, and I saw no ill effects from that replacement. In theory I could've run slightly lower pressure on that tire to help compensate, but I didn't go out of my way to do so.

 

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Of course the difference in size front to rear is not important if the vehicle is just a two wheel drive. Another thing happens when tires on the drive wheel are subject to hard acceleration as the one with the most torque pull will wear much more. A good reason for rotating.

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You sound like you live in the technicalities of life. However, your opinion and basic geometry a very different. Also, batch differences in manufactured tires provide negligible effect on any aspect of a tire.

What you’re suggesting, measuring the radius of a tire that’s inflated and deformed under load as a way to approximate rolling diameter, is janky advice at best. Working with the dimensions of a tire before it’s mounted, inflated, and then deformed under the the load of the vehicle is the most ideal (and correct) way to get the right tires.

 

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Yes, I am being technical. What you are saying is extremely simplistic which is common because of common presumptions. Show me two worn tires that are identical.

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Yes, because the simplest way to solve a problem is usually the correct way. You see that’s the beauty of math. When you strip away the useless details, you’re left with your answer. I’m not sure how the basic geometry of a tire is extremely simplistic, want to expand on that? 2 identically worn tires still won’t validate your terrible advice by the way. Whatever irrelevant variables or technicalities you have are just that, and they help no one make the correct choice

 

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Two points: First, as to stripping away "useless" details in math means you may be leaving out important factors. If so, you will not get the correct results. It's kind of like the proposition that humans are causing climate change "proven" by computer inputs that are incomplete.

Second is that you are wrong, two tires that are identically worn more likely prove that my advice is correct. Although I'm not trying to prove anything to anyone about what they do to their tires, including with pressure. I'm just pointing out the variables that may affect the outcome of wear. I'm just giving some insight as to what the variables are.

Just because you are limiting your vision to my facts as being "terrible advice", then that's just your opinion. Others can consider what I said, and, even test anything I said for themselves.

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Thank you all for the replies.I would have thought that just changing the width, wouldnt have changed the overall diameter.According to this calculator, it does.Not sure why that is.Thanks for the info.

 

Ive decided to put the same size tires on the rear as what I have on the front.Hope this help somebody else wondering if you can put all the same size tires on this car.