Mis-matched spare?

[eddiebx]

Boy oh boy. I didn't just say "Yeah, it's fine". I said "As long as it is just temporary, there won't be any devastating results." Am I wrong?

I mentioned temporary use after quoting a comment about a donut. I will admit that I assumed a certain level of common sense from the OP. I didn't think it was necessary to explain the limitations of a donut spare.

Toyota uses donuts for a number of vehicles. Having a full-size spare is obviously ideal and is considered a premium. I can assure you that the price for that full-size spare is well compesated for by the consumer. I didn't in any way insinuate that using a donut or mismatched tire (minimal in the op's case) as a great idea.

Again, it is fine in a temporary (less than 50 miles, under 50mph) type of situation (on surface streets, not rock crawling). Just ask Toyota.hwell:

Pretty sure you'd never get anything close to an official okay from Toyota in writing; ask them and let me know if you do. I'll gladly shove my own foot in my mouth at that point

All I have to say is that having TCS/VSC kick-in when it shouldn't is no laughing matter. For example, a while ago I believe my zero-point calibration was out of whack and during a hard panic stop on a completely dry road, the computers decided that I might be flipping over (or something to that extent) and kindly let off the brakes for me in order to modulate them (increasing my stopping distance significantly). Now imagine if I didn't leave enough room in front of me? Stomping on a brake pedal and helplessly rolling forward is not a fun experience.

Greg's point (and mine as well) is, there's a lot more electronics on these newer vehicles nowadays, all of which are working in harmony. These electronics rely solely on sensors to provide an accurate representation of the current vehicle dynamics, and use algorithms to make an educated guess as to what's going on and how to respond properly to keep you out of trouble. Messing around with how the sensors gather their data and providing false readings to the computers can only lead to no good at some point. It's really not a matter of if, it's when.

Now do you really want to find out at the worst possible moment exactly how much tolerance is programmed into these algorithms?

Sadly the days of a car having a gas pedal for going and a brake pedal for stopping are long gone.

 

[1engineer]

Pretty sure you'd never get anything close to an official okay from Toyota in writing; ask them and let me know if you do. I'll gladly shove my own foot in my mouth at that point

All I have to say is that having TCS/VSC kick-in when it shouldn't is no laughing matter. For example, a while ago I believe my zero-point calibration was out of whack and during a hard panic stop on a completely dry road, the computers decided that I might be flipping over (or something to that extent) and kindly let off the brakes for me in order to modulate them (increasing my stopping distance significantly). Now imagine if I didn't leave enough room in front of me? Stomping on a brake pedal and helplessly rolling forward is not a fun experience.

Greg's point (and mine as well) is, there's a lot more electronics on these newer vehicles nowadays, all of which are working in harmony. These electronics rely solely on sensors to provide an accurate representation of the current vehicle dynamics, and use algorithms to make an educated guess as to what's going on and how to respond properly to keep you out of trouble. Messing around with how the sensors gather their data and providing false readings to the computers can only lead to no good at some point. It's really not a matter of if, it's when.

Now do you really want to find out at the worst possible moment exactly how much tolerance is programmed into these algorithms?

Sadly the days of a car having a gas pedal for going and a brake pedal for stopping are long gone.

Eddie's preachin' the word! I have nothing to add that's not been said. Bottom line, stay with same same tire size, or next time define temporary.

 

[KCcarguy]

I would go for the same size and load range, but I wouldn't worry about the brand/ model matching. Don't go with "nearly-bald". You don't want to rely on that if you have a flat out in the middle of nowhere. Just be patient/ keep your eye out and you'll find a good used spare on CL.

 

[LandCruiser]

I'd never do it in a modern 4WD (or even a 2WD truck) with all the new safety systems, and computer controlled stuff. I'd have no idea if the front wheel hubs are engaged, if the truck thinks the smaller wheel is spinning faster, whether I'm tearing a diff or a shaft into pieces.

The last time I did it was in an FJ60 in 2WD mode. I had a flat in the back and no spare. I took a front tire off, mounted it in the back, then put the tiny spare in front (with manual lockers unlocked). Even then I was worried something might happen and after that ran a cheap used spare 35" instead of the tiny spare 32" to match everything.

 

[enduro]

When I was concerned about this same question when I bought my AWD Limited 4R and went aftermarket wheels/tires.

From the information I have gathered (independently) as well as talking to a Toyota technician, the consensus is the 4R's Torsen center diff and computer has a tolerance of 3% of tire difference from front to rear.


You have an SR5, so this is even less critical as you are not 4wd all of the time.

What size tires/wheels did you go with? Based on your statement above did you end up replacing your spare tire/wheel?

Cheers

 

[FatTiresRollin]

I ended up scoring a used Duratrac from Craig's list for $90. Same size of course as the other 4 at 285/70/17. The advice from the forum members was overwhelmingly in the direction of having a matching spare. I fully believe that an undersized spare would be OK in an emergency on-road situation for a short distance. But the "unknown" is what influenced and ultimately made my final decision...which in the end is really the best solution.

 

[Jhalko]

On the jeep GC I had before my 4runner I had full time 4x4, I went up just ONE size and left the spare. Of course got a flat out in Utah on a mtn biking trip. Drove home with the different sized spare, the dash flashed errors and annoyed me the whole drive home. Dangers and drive ability aside having the dash electronics flashing was so annoying I had wished I changed all five.

I gotta agree with others, no reason to compromise safety on something you can control.

 

[NorthWest 4R]

What size tires/wheels did you go with? Based on your statement above did you end up replacing your spare tire/wheel?

Cheers

I went with Duratrac's in 265/70/17 on method race double standards with 1.25 spidertrax's. I am using the 20" LTD stock spare (painted black) as they are VERY VERY close to the Duratrac's overall diameter (I measured personally, but don't remember the #'s)

Hopefully I never need it, I also carry tire plugs and air. So that would be quicker than changing a spare, as long as its repairable. I will be going bigger size tires later on but want to wait until I have all my suspension done so I can see my max tire size available. Also build a rear bumper/spare tire carrier, then I will get a matching wheel/tire for the back.

 

[enduro]

I went with Duratrac's in 265/70/17 on method race double standards with 1.25 spidertrax's. I am using the 20" LTD stock spare (painted black) as they are VERY VERY close to the Duratrac's overall diameter (I measured personally, but don't remember the #'s)

Hopefully I never need it, I also carry tire plugs and air. So that would be quicker than changing a spare, as long as its repairable. I will be going bigger size tires later on but want to wait until I have all my suspension done so I can see my max tire size available. Also build a rear bumper/spare tire carrier, then I will get a matching wheel/tire for the back.

Thanks for the info.

 

[proquip]

I've decided to chime in on this topic as I'm currently going through the process of deciding what to do with my spare. I'll start by saying that I'm making general assumptions about a possible scenario that might explain the allowed variance (as designed by Toyota engineers) while avoiding any undue impact to the vehicle's drivetrain or electronics (VSC, etc.). I have no inside knowledge of how the vehicle was actually designed.

For me, the best example of allowed variance between two tires would be an as-new stock spare and a mostly worn stock tire down to the minimum recommended 4/32 tread depth (used to be 2/32). For the stock 4Runner, we have a 265/70R17 Dunlop or Bridgestone tire that measures 31.7" in diameter with 11/32" of tread depth when new. If your tires were worn down to the 4/32 tread depth, you would have tires that are now 31.26" in overall diameter (see below).

31.7" - [2 x (11-4)/32] = 31.26"

So, you could end up running a 31.7" diameter tire (spare) and a 31.26" diameter tire (worn) at the same time. If you apply that ratio (31.26 / 31.7) to an oversized tire setup, in particular the 32.8" diameter tires I just ordered (285/70R17 tires generally measure 32.8" when new but some measure a bit more, like Duratracs or KM2s), I calculate that your spare tire could measure 32.3" in diameter and still keep to the above ratio.

32.8" x (31.26 / 31.7) = 32.34"

For me, being a fan of pizza cutters because the spare would tuck in a bit more than the current spare, I'm seriously considering either a worn 255/80R17 BFG KM2 (32.4" at 4/32 tread depth, but just haven't seen any for sale in my area), or perhaps a new Goodyear MTR Kevlar in 255/75R17 which runs a bit tall at 32.3". As my new tires wear down, the variance between the two possible tires actually narrows, which is a good thing.

I've seen a lot of debate on this topic in the past, but I wanted to put some logic and numbers to see where I landed. I think the options I'm pursuing would be consistent with the worn tire / new spare scenario that in my mind must have been factored with the drivetrain and electronics for the 4Runner were designed. Then again, what do I know?!?