Spare Tire Off Size

[Lucky200K]

Had a question on the spare. I upsized from the OEM 225 70x16 tires to 235 70x16, but did not change the spare. If I have to use the spare at some point, is this minimal difference in diameter going to cause any transmission/differential problems if I go for any significant amount of distance?

I'm thinking it won't if I keep it under say 50 miles, but I don't really remember the answer to this question.

[boRXis]

You didn't change your diameter - 235 is a wideness(mm). It won't hurt you.

[Lucky200K]

:)

Thanks, I know 235 is the width, but the aspect ratio is still 70 so the diameter has increased marginally. I guess the issue wouldn't be any different than using a new spare on an axle with a tire that's almost bald. The diameter difference would probably be about the same.

Hopefully I don't blow a tire soon...

[RX in NC]

A new 235/70/16 tire is approximately seven-tenths of an inch larger in diameter than a new 225/70/16 tire. Engineers will tell you that if you run one slightly smaller (or larger) tire as compared to the other three, you definitely run a greater risk of transmission/differential damage over a period of time. The length of that period of time remains in question and probably depends upon your driving habits and general road conditions in your area.

Like you, when I upsize tires I typically don't upsize the spare, thinking that if I ever have a flat and have to put the spare on, it won't be on for long (I've never had to do that, knock on wood). On my 1999 Dodge Ram I'm now running 275/70/16's whereas my spare is the original never-used 245/75/16, and the diameter difference between those tires when new is a whopping 2 inches.

But knowing what I've been through with regards to the inherent weakness of the RX transmission, this is one vehicle that in your case I would probably upsize the spare tire on. If you decide not to, I'd recommend that you never drive the original spare for longer than it takes to go directly to a tire shop and get your 235/70/16 damaged tire repaired or replaced. On the plus side, as you drive on your mounted tires the difference in diameter between them and your spare will gradually decrease as your mounted tires' tread wears out.

I'll be replacing the OEM tires on my wife's RX late this year, and I've decided to stay with the 225/70/16 size as I move from Bridgestone to Yokohama. I just don't trust the RX transmission enough to upsize the tires from the original specs.

[Lucky200K]

:P

I've actually upsized for almost a year now and only drawback is the slightly lower mpg (about 1 mpg drop avg), not that the RX had much mpg to give. Other than that, I feel the car handles better now and the tires clear the wheel well.

Thanks for your post, I will hopefully never need to use the spare, but if I do I'll just use it enough to make it to next exit to change the tire.

;)

About your comment on the RX transmission, I know it's been a topic of posting on this forum and others, but really... how many RX have this issue? Forums are naturally for owners who have problems and come on to post, but out of all the RX sold, how many really had this problem?

[RX in NC]

It's hard to say how many RX owners have experienced transmission problems, but the complaints about it are widespread and well-known across the U.S. and Canada. Lexus dealerships are well aware of the complaints and have been trained to deflect them with comments such as "the transmission is performing as designed". Our own experiences with my wife's 2000 RX have caused us to decide that we'll never buy another one. Her transmission was replaced under warranty last November at about 48,000 miles and while she no longer feels unsafe in her vehicle, it still shifts more like a $12,000 vehicle than a $35,000 one.