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you & your car

I have purchased a 2010 Toyota Venza with 20-inch rubber. Last month, the Toyota dealer had a bunch of us in for a captive sales pitch for service, aftermarket stuff and winter rubber. I asked if they had steel rims at 20 inches for the winter tires and they said no. They had 17-inch steel rims and said we would have to use them. What are we doing here? Is this not over the safety boundary that you mentioned in an earlier column? – Spd in St. Catharines, Ont.

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This is the most frequently asked question in my inbox and one that tire stores are inundated with.

Two things are at play here:

  1. Toyota sells a lot of cars/vehicles so the demand for winter or replacement tires is high.
  2. The Venza, Highlander and Camry are all built on the same platform or underpinnings. In other words, they are all alike under the skin, with similar engines, transmissions, brakes – and tires.

The latter is where the answer to your question lies – and the reason the dealer has 17-inch steel wheels that will fit.

Assuming the wheels fit over the brakes and have the proper bolt pattern and offset – i.e. the centerline lies in the same exact spec, 16-, 17-, 18-, 19- and 20-inch wheels from or designed for the Camry, Highlander or Venza will fit and work.

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The 16-inch steel wheel that is standard equipment on the least-expensive Camry has the same offset and bolt pattern as the 20-inch wheel on your Venza.

But the next step is where safety issue comes into play – what size tire you install on those wheels or rims. We have the offset, bolt pattern and brake clearance covered, but wrapping the wheel with the wrong-size tire can create huge problems.

The issue is the overall diameter of the wheel/tire combination. This in turn dictates how many times or revolutions the tire and wheel will turn in any given distance, expressed in revs per mile or kilometre.

A great many dynamic functions are keyed to the revs per mile. These range from very important things like ABS, electronic stability control and handling to non-safety issues like fuel mileage, speedometer accuracy and even automatic shift transmission shifts. During the development of the vehicle, engineers specify the revs/km of the tire with all these factored in – so changing that circumference can be dangerous.

It is generally accepted that if you keep within 1 to 3 per cent you are okay. The earlier example you were referring to was outside that range.

To put numbers to the extremes of this Toyota size issue:

The 245/50R 20 tire that came on your Venza has an overall diameter of 752.85 mm, sidewall height of 122.4 mm, circumference is 2,365 mm and turns 423 times every kilometre.

A 245/65R 17 tire on the steel wheel recommended by your dealer has a diameter of 431.8 mm, circumference of 2,356 mm and turns 424 revs/km.

That will result in a slight, but acceptable, speedometer error. When it reads 100 you will actually be travelling at 99.8. That is not an issue since there is not a speedometer in the industry accurate to within that 0.2 per cent error.

The short answer is yes, you are okay with the 17-inch wheel and a proper-size tire.

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