4WD Question for Our Beloved Runner Nerds

[UFG8R Runner]

So I swapped out the factory tires for 275/60R20's on stock rims.

Let's say I get a flat tire...will swapping the (still factory sized) spare in with 3 other tires that are a different size cause problems with the 4WD / transfer case / etc?

Thanks in advance.

 

[Qjake]

I drove about 100 miles on the highway doing 75 with my 275/55r20 spare on the right rear mixed with my 295/65r18's no issues.

Something else to consider, How many cars on the road have those little donut spares? You'll be fine.

 

[UFG8R Runner]

I'm definitely not an expert on 4WD systems, but I remember the mechanic at the dealer for my 1997 Jeep Grand Cherokee Quadra-Trac telling me that if I put a mismatched spare on it would burn up the AWD system. He claimed I needed to call a wrecker if I ever had a flat because I had a mismatched spare.

I don't know if that was true, but it sounded legitimate. And I wondered if it is an issue with 4Runners.

He claimed the system would be trying to push power to the tire that is rotating at a different speed because it thinks its slipping.

 

[Qjake]

Yeah :shrug: just sharing my experience. Whether right or wrong I dunno but I had no ill effects.

 

[Okki]

from Cars.com:
Small spares can damage the vehicle, too
Important: Small spares can damage the vehicle itself if used for too many miles. For two tires to work together on the same drive axle requires a differential, the job of which is to let the wheels turn at different speeds. When the vehicle travels in a straight line, the differential is effectively dormant. If one wheel is smaller than the other, however, the differential is working constantly, for which it was not designed. Driving 200 miles on a small spare can put the equivalent of tens of thousands of miles on this set of gears, causing premature wear. Certain all-wheel-drive systems also can sustain damage from this size mismatch. Follow your owner's manual's instructions carefully.

Small spares, speedometers and safety equipment
Small spares may undermine a car's speedometer operation and such safety features as ABS, traction control and electronic stability control. All these systems depend on equal rotation of all the wheels in straight-line driving, often using wheel-rotation sensors that are calibrated to work with a full-size tire. A smaller tire may throw off the speedometer reading or disable one of the other systems, all of which affect safety.

Your owner's manual should explain if your car may suffer these consequences when employing a small spare tire.
-----------------------------

Short distances at slow (usually limited to 50 mph) are generally fine, but something to consider; most insurance's offer a free towing service (USAA does for instance) and if you are still under your Toyota Care package, that includes a tow as well.

Beyond that... get a same size spare and tire-rotate in with the rest.:hungry:

Okki

 

[Ducmonsta]

If its AWD, the overall diameter needs to be the same. The width does not matter. The tcase has a differential in it. In a JGC, it's called a viscous coupler. Very well known to go bad. It's the same if you have a limited slip in the rear. That limited slip will always think the tire is slipping if the tires are different diameters. It will burn up the clutches in it.

 

[Qjake]

Good info, guess I won't be doing that again! :ybrick:

I bought 5 of each when I got the new wheels and tires so I'm good now anyhow.:bigok:

 

[cbielat]

I have been wondering about this as well. I am running four 285/70/17 and still have the stock full size spare. Correct me if I am wrong, but wouldn't the stock spare tire size, as the numbers show, be the same height tire as the 285? The reason I want to think this is because 285 being the width is wider but the 70/17 be the same?

 

[DannyLlama]

I was told by my local tire shop that it will even impact the transmission, not sure how true that is, just sharing/plaing "phone"

 

[phillyd2]

Add to this the fact that your tire won't go flat in the parking lot of a tire store but at least two hours from the nearest pavement. Buy the correct size.

 

[STLDAVE]

I have been wondering about this as well. I am running four 285/70/17 and still have the stock full size spare. Correct me if I am wrong, but wouldn't the stock spare tire size, as the numbers show, be the same height tire as the 285? The reason I want to think this is because 285 being the width is wider but the 70/17 be the same?

Actually the 285/70r17 is about an inch taller than a 265/70r17. The reason is the "70" part of the description. It indicated the ratio of height (wheel rim to tread) versus the width of the tire, in this case the tire height is 70% of the width. So, as the tire gets wider, the height does too. Here's a good link to see how changing any if the numbers in a tire size changes the dimensions of the tire.:

Tire Dimensions Made Simple - Discount Tire

 

[1engineer]

If its AWD, the overall diameter needs to be the same. The width does not matter. The tcase has a differential in it. In a JGC, it's called a viscous coupler. Very well known to go bad. It's the same if you have a limited slip in the rear. That limited slip will always think the tire is slipping if the tires are different diameters. It will burn up the clutches in it.

Duc is right as rain as usual!

I have been wondering about this as well. I am running four 285/70/17 and still have the stock full size spare. Correct me if I am wrong, but wouldn't the stock spare tire size, as the numbers show, be the same height tire as the 285? The reason I want to think this is because 285 being the width is wider but the 70/17 be the same?

"285" is sidewall height in mm
"70" is width of tire in a percentage
"17" is wheel dia

I was told by my local tire shop that it will even impact the transmission, not sure how true that is, just sharing/plaing "phone"

Yes Danny, your tire shop is right. Here is what can happen if you run "mismatched" tires on a 4WD or AWD:

Your transmission, brakes, differential and all safety electronics such as VSA and ABS could be affected.

Electronically, there is a device that counts rotations of each wheel and that data is fed back to the computer. If that wheel is outside the rotational normal margin of error because you put a too large or too small tire on the vehicle then ABS and VSA do not know what to do and will think that wheel is slipping and could apply the brake or re**** the throttle. Not good especially in an emergency situation.

Mechanically, a mismatched tire could, as correctly stated above, cause limited slip conditions within the drivetrain and burn up the differential over time.

With that said, there is a margin of error for different rotation. When you make a turn, your outside wheels have to go farther than the inside turn wheels to go the same distance so rotational speeds are different. Same thing if you have a worn tire and a new tire of the same size. There has to be a margin of errror but I do not know what that is. Hope this helps explain things.

 

[Zeder]

I would be more concerned if you had the AWD setup though. With the part time 4WD drivetrain in the Trail, as long as you are NOT in 4WD the transfer case is not in play and the rear diff is open (right?). So there should be absolutely no issue with respect to mechanical trauma. 1engineer makes many good points about the other systems that could be affected though.

Edit:
I would think that the Trail goodies like A-trac and Crawl Control would get very messed up though. These options rely heavily on rotational speed of each wheel as inputs to their programs.
Z

Sent from my mobile. Probably from the middle of nowhere...

 

[DannyLlama]

Same thing if you have a worn tire and a new tire of the same size. There has to be a margin of errror but I do not know what that is. Hope this helps explain things.

That's right he also did say that if the tires have been worn down to half their thread, I would have to replace all of them (in the event of a flat). Due to the "full size" spare having full thread life. Which I didn't understand the point of having a full size spare at that point, I guess still don't. Not to mention how uneconomical it would be to replace all four tires because one flat.
End rant :nerd:

 

[1engineer]

That's right he also did say that if the tires have been worn down to half their thread, I would have to replace all of them (in the event of a flat). Due to the "full size" spare having full thread life. Which I didn't understand the point of having a full size spare at that point, I guess still don't. Not to mention how uneconomical it would be to replace all four tires because one flat.
End rant :nerd:

Not true. As I said in the above post there has to be a +- percentage of error for turns and tire wear. I would imagine tires could go to wear bars vs new tires before any issues present themselves. Another thing is even though tires SAY they are a size between brands they will be dimensionally different because of tread depth and number of plys for example.

 

[janefennell147]

I won't be doing that again

 

[UFG8R Runner]

Thanks for all the great responses, guys!!!

Soooo, anyone happen to know if a 275/60 will fit in the factory spare location?

 

[Qjake]

Sure will :boink: