Switching to 235/50/R18 tires.

[Jason Alex]

I have a 2007 RX 350 with 235/55/R18. I was thinking of going with a 235/50/R18. Has anyone tried this? Would this be an issue?

 

[Lexigrl]

It would likely cause issues. The difference in diameter is 1.1 inches smaller or 3.3 percent. You want to keep the variation less than 3 percent. Beside your speedo being off, the car's computers likely won't like it either unless the new size can be programmed into the car. Not sure that was possible for a 2007 model year car. Might have to ask the dealer if that's an option to reprogram for different size tires. Whenever I have wanted to change from stock size, I go to discount tire and see what "plus zero" options are available (if any) and go from there. 

[RX400h]

You'd have to go to a 255/50/18 (28.04 inches diameter) to get close to the 235/55/18 (28.18 inches diameter)

Here's how you calculate it:

Example: 255/50/18 tire
18 + (255x.50x2/25.4) = 28.04 inches

18 + (235x.55x2/25.4) = 28.18 inches

Of course, you'd have to find out if the wider tires would fit.
Why do you want to change?

[Jason Alex]

12 hours ago, Lexigrl said:

It would likely cause issues. The difference in diameter is 1.1 inches smaller or 3.3 percent. You want to keep the variation less than 3 percent. Beside your speedo being off, the car's computers likely won't like it either unless the new size can be programmed into the car. Not sure that was possible for a 2007 model year car. Might have to ask the dealer if that's an option to reprogram for different size tires. Whenever I have wanted to change from stock size, I go to discount tire and see what "plus zero" options are available (if any) and go from there. 

 

Thanks for the tip.  I did go to the discount tire website and 235/50/r18 is an option in the +0 category, however it is listed as undersized and may or may not fit on the wheel.  It looks to me as it would fit.  My concern is if there are any legitimate performance or handling issues that could be a safety concern.  

 

9 hours ago, RX400h said:

You'd have to go to a 255/50/18 (28.04 inches diameter) to get close to the 235/55/18 (28.18 inches diameter)

Here's how you calculate it:

Example: 255/50/18 tire
18 + (255x.50x2/25.4) = 28.04 inches

18 + (235x.55x2/25.4) = 28.18 inches

Of course, you'd have to find out if the wider tires would fit.
Why do you want to change?

There are 2 reasons.  I'd like a little bit of a lower profile, but not too much.  And ease of availability here locally.  

[RX400h]

 I forgot to check availability of the 255 tires. It looks like only Pirelli has them as a Summer tire and they are extremely expensive. Discount Tire will order tires and receive them within a week. They rarely keep stock of the tires I buy.
Honestly, I agree with Lexigrl that going with a much smaller diameter tire is risky.

[Jason Alex]

6 hours ago, RX400h said:

 I forgot to check availability of the 255 tires. It looks like only Pirelli has them as a Summer tire and they are extremely expensive. Discount Tire will order tires and receive them within a week. They rarely keep stock of the tires I buy.
Honestly, I agree with Lexigrl that going with a much smaller diameter tire is risky.

At this point I will probably just stick with the 235/55/r18 tires.  With the 235/50/r18 tires, I figured there wouldn't be any handling problems because of the lower profile tire and the lower center of gravity.  In face I would think handling would be improved.  I think the only downfalls would be the speedometer being off and increased risk of damage to the wheels from potholes because of the reduced thickness.  

[RX400h]

You can always call the pros at tirerack and ask them what they think. If you had a 1990s-era vehicle, I'd be less concerned but there are so many safety-related electronics in today's vehicles that a tire diameter change could cause issues of which we do not consider.

[Jason Alex]

On 3/26/2022 at 1:16 PM, RX400h said:

You can always call the pros at tirerack and ask them what they think. If you had a 1990s-era vehicle, I'd be less concerned but there are so many safety-related electronics in today's vehicles that a tire diameter change could cause issues of which we do not consider.

Ah that's good advice.  I have emailed them and will probably call them tomorrow to see what they say.  Thanks.  

[Jason Alex]

Update.  I received an email from Tire Rack:

Tire Rack Sales <saleshd@tirerack.com

 

Thanks for your inquiry.

 

This would not be recommended.  The tires would have a lower load capacity with the smaller air chamber. They may not carry the weight of the vehicle safely and would wear out quicker.

If you have any other questions or would like to place an order, please feel free to contact me. If you are placing an order online, please be sure to reference my name on the order form in the box directly under the credit card number, so that I can watch over the order and expedite the process.

 
Best regards, 

 

-Parker

[RX400h]

Yes, load capacity is very important, also.