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I have a 2004 Lexus RX330. The tire label sticker on the driver side indicates that the size of tire that should be used is a 225/65 R17. However, when purchasing tire options on various retailers, they all indicate that there are multiple sizes, not just the one listed before, but another size, P235/55 R18. Then I looked at my tires and sure enough, I've been using the R18 size for at least a few years without noticing!

A mechanic I trust told me to never use a website to verify tire sizes, and instead rely on what's listed explicitly on the stickers on the car. So what's with this weird situation? Why would the sticker on the car only list the 225/65 R17 size, while online websites list 2 possible sizes?

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    Does your speedo read true? IE when you're doing the speed limit on a main road, are you going noticeably faster than the crowd?
    – Criggie
    Mar 14 at 2:49

4 Answers 4

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When the car was originally purchased there will have been different options for the alloy wheels. The alloy rims will have been either 17 inch or 18 inch in diameter. There may even have been different designs in both sizes of rims.

Clearly a 17 inch rim needs a 17 inch tire (225/65 R17) and the 18 inch rim needs a 18 inch tire (235/55 R18).

There is also the possibility that the car was purchased with the 17 inch rims and that an owner has swapped them for the 18 inch rims. This would explain stickers specifying 17 inch tires, yet your car has 18 inch tires.

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  • I see. So should I rely on the sticker and try to go back to a R17 sized tire? Or stick with R18? Is there a benefit for one over the other?
    – yuritsuki
    Mar 13 at 10:09
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    It is really just cosmetics, so a personal choice. To go back to 17” tires you would have replace your rims which may be expensive. It may be that 18” tires are a lot more expensive than the 17” tires so it may be worth the change.
    – HandyHowie
    Mar 13 at 10:34
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    Is it really just cosmetics? In earlier times a change in wheel diameter would affect the accuracy of the speedometer/odometer. Is there a sensor or switch nowadays that compensates for this?
    – Jim Mack
    Mar 13 at 14:42
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    @JimMack no, changing the tire outer diameter does that.
    – hobbs
    Mar 13 at 15:35
  • @JimMack as hobbs points out, the 18” rim takes a lower profile tire (55%), so the circumference of the wheel will be very close to that of the smaller rim with a higher profile tire (65%)
    – HandyHowie
    Mar 13 at 16:17
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Short version: your car is OK with these tyres and rims.

Long version:

What really is important for the car is the external geometry of the wheel.

How much of the wheel is the rim and how much is the tire can affect the comfort or the handling, but the car doesn't care.

  • Diameter:

Your 2 variants (225/65/R17 and 235/55R18) differ by 7mm or 8mm in diameter (in theory) for a ~70cm wheel. This is about 1% of the size. Tyres wear more than that before they need replacement and they are not made that much accurate in size in the first place. No issue here.

  • Width:

The width of the wheel can be another issue. If the rims have equal offset, this makes the wheel 5mm wider at each side. This may make the contact between the wheel and parts of the suspension or the arc more likely, when the suspension is deeply articulated and the front wheels are steered.

Most of the cars are engineered to tolerate way more wheel size variations - first, because not all 225/65R17 (or whatever) are made equal and second because of the possibility to use snow chains. Snow chains can add up to 30mm in each direction (and more if you are novice and don't tighten them properly).

In short, you may not be able to fit aggressive snow chains. Since you are asking the question, this is probably not an issue for you.

  • Regulations

In most jurisdictions, you are not allowed to alter important parts of your car without a proper regulatory oversight.

These things are enforced to a different degree at different places.

Depending on where you are, it may be that no one cares. Or it may be the car to be considered non-road-legal and the driver fined if the required certificates are not presented.

Or something in between.

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  • I was going to comment that tyres are normally supplied with 8mm of tread and we usually replace when we reach 3mm, which is just 5mm of wear. But then I realised that's radius, so tyres actually wear by about 10mm in diameter, and your claim is correct. Mar 13 at 17:31
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From year 2005 (different year, but same generation) sales brochure:

enter image description here

Both wheel sizes are possible and allowed.

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  • Uh… what? How can the wheel/tyre diameter not change the milo/speedometer readings? Mar 13 at 21:46
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    @RobbieGoodwin Because the tire profile is different between the 17” and 18” wheels, so that the resulting circumference is very similar. The speedo will then be correct for both sizes.
    – HandyHowie
    Mar 13 at 22:24
  • @HandyHowie Is that your way of saying that although the wheels are different sizes, so are the tyres, and those differences average out… or what? Mar 13 at 22:26
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    You could put it like that. One one wheel you have a small rim and a thick tire, on the other wheel you have larger rim and a thinner tire. The overall diameter and circumference is about the same for both.
    – HandyHowie
    Mar 13 at 23:09
  • @RobbieGoodwin most of online tyre calculators have an option to compare two tyre sizes. If you input both sizes mentioned in the original post you can see that speed wise difference is negligible (59.4mph vs 60mph).
    – Alex
    Mar 14 at 6:50
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The wheel well of your vehicle has enough clearance to accommodate the turning (left/right) travel of both tire sizes as well as the suspension (up/down) travel of both tire sizes.

The tires and wheels physical dimensions are similar enough.

225/65 R17

  • the wheel is 17in, or 431.8mm in diameter
  • the sidewall of the tire is 65% of the width, or 146.25mm.
  • so the diameter of your tire/wheel combo is 146.25mm + 146.25mm + 431.8mm = 724.3mm
  • the total circumference of your tire = 2,275.5mm
  • the width of your tire is about 225mm

235/55 R18

  • the wheel is 18in, or 457.2mm in diameter
  • the sidewall of the tire is 55% of the width, or 129.25mm
  • so the diameter of your tire/wheel combo is 129.25mm + 129.25mm + 457.2mm = 715.7mm
  • the total circumference of your tire = 2,248.5mm
  • the width of your tire is about 235mm

Your 225/65 R17 is actually a little bit bigger around than your 235/55 R18.

Why have an 17in and an 18in choice? Well, the 17in tire, with an aspect ratio of 65 means the sidewall of that tire is taller and has more rubber. This can be equated to a softer ride. The 18in tire, with an aspect ratio of 55 means the sidewall is shorter with less rubber. This can equate to a stiffer, but possibly more performance-aggressive ride. Purely a personal choice to go with more rubber and comfort, or less rubber and more metal.

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