Small OE spare on AWD vehicles

[Ronzuki]

That would be OK for the AWD aspects, but it would change the measured speed (speedo) and the Odometer elapsed mileage.

It's going to affect a whole lot more than that...all of the automation requires known ratios for wheel speed...you're thinking of the speedo being the primary concern as in pre-techno-marvels days. There's absolutely no way for the common owner knowing what effects changing tire sizes would have upon the numerous control algos. I can almost guarantee that 'exception' wasn't handled in the OEM's programming (software? firmware?). Tire size changes had barely been considered on JK models of Jeep Wranglers, when nearly everyone changes (increases) tire sizes, and many also alter the R&P ratios. Dealers eventually could recal tire size, only to a point. It would sorta correct speedo, no clue regarding any of the other systems reactions. The '04 TJ Rubicon I had was off, brand new from the factory. They never installed the correct speedo gear for the 4:1 transfer case. Clocked on more miles than actually driven, which good for them, caused warranty to expire sooner than it should have, or would have, had it been a standard model TJ.

...dumb to have such a discrepancy in tire diameter when the spare cavity is clearly designed to accommodate the correct diameter.

Repeat....build dollars for added techno-BS has to come from somewhere...

 

[jaycal6]

Just bought an older 18x4 CX-9 wheel. Tire is probably too old to use, but I’ll grab a new Space Miser.

I know it’s not ideal, but will it be possible to install that on the front of a 2.5T in a pinch?

 

[sm1ke]

I don't think Mazda North America provided us with a spare that would damage the drivetrain. In particular, the manual does not say how far you could drive with the spare on. Sure, the spare is skinny and will not last for very long, but it seems that it will not damage the drivetrain as well.

I agree with murky in that that is a dangerous assumption. The manual doesn't specify a distance, but it does state that the car should be driven straight to a tire repair shop. Direct quotes from the 2014 CX-5 manual:

"This tire should be used only for an emergency and only for a short distance.
Use the temporary spare tire only until the conventional tire is repaired, which should be as soon as possible."

"The temporary spare tire is lighter and smaller than a conventional tire, and is designed only for emergency use and should be used only for VERY short periods. Temporary spare tires should NEVER be used for long drives or extended periods. "

Additionally, based on the quote from the ClubLexus post earlier ("An AWD drivetrain has a lot of rotational mass. It's easy to upset the balance"), upsetting that balance will make the related systems work harder to correct that imbalance. Example from the ClubLexus forum again:

"For instance, wider rear wheels on an Audi quattro will resist more than the front wheels. The differential will think that the grip-difference is a slip-angle difference, and send more power/torque to the front to compensate. You'll end up with a permanent front-biased power/torque distribution. This will not only make the car understeer, it makes the wider rear tyres completely useless b/c the more grip they have, the less power/torque they'll get."

This isn't what would happen in your case because on the Audi, the width of the rear wheels is different, while we're talking about wheel diameter. It's just an example to show how wheel size can affect the rest of the car.

As for the statement from owner's manual that "do not install the temporary spare tire on the front wheels (driving wheels)", I'd take it like many other questionable info in the owner’s manual, such as no maintenance schedule on ATF and brake fluid, and wrong fluid capacities on engine oil, rear differential, and wrong bulb spec on rear turn signals.

If I have a flat at front on the highway, I definitely will put the compact spare on the front and drive my CX-5 to a nearest tire shop to get the repair. Changing the tire once on the highway shoulder is dangerous enough, especially with those 18-wheelers constantly passing by!

I understand your misgivings with some of the other things in the manual, but putting the spare on the rear isn't "questionable information". See below.

"Driving with the temporary spare tire on one of the front driving wheels is dangerous. Handling will be affected. You could lose control of the vehicle, especially on ice or snow bound roads, and have an accident. Move a regular tire to the front wheel and install the temporary spare tire to the rear."

There are reasons for why they recommend putting a spare on the front. If you're more comfortable putting the spare on the front to get off the highway, well, it's your car and you'll do whatever you want to do in the end. But Mazda's recommendation is not "questionable" or wrong, it's just different from your personal preference.

It definitely performed worse with the spare in the back. I cannot imagine how bad it would have been on the front.

Mazda agrees with you too.

 

[sm1ke]

Just bought an older 18x4 CX-9 wheel. Tire is probably too old to use, but I’ll grab a new Space Miser.

I know it’s not ideal, but will it be possible to install that on the front of a 2.5T in a pinch?

That would depend on backspacing and offset of the 18x4 wheel. It needs to have enough space to clear the brake caliper. Any chance you have those numbers handy?

 

[jaycal6]

That would depend on backspacing and offset of the 18x4 wheel. It needs to have enough space to clear the brake caliper. Any chance you have those numbers handy?

Hmm, unfortunately no. Guess I’ll just see how it goes.

 

[Mazdiod2]

It seems a shame to devote the space in the trunk for something you (might) never need.

I guess thats why some makes have tried replacing the spare with a tire pump and a can of fix a flat. That and a cellphone might be all most people need.

 

[sm1ke]

I keep a booster pack, fire extinguisher, first aid kit, and a few other things that I may never need in the trunk. It's rare that I'd ever need to use any of it, but it's there when I need it. I'd rather have it than be caught without it.

I think most makes also offer Roadside Assistance (sometimes included with an extended warranty or similar). If you have that, technically you could remove all of the extra stuff, including the spare and the emergency jack. But then you'd have to sit on your hands while you wait for the assistance to arrive.

 

[yrwei52]

Just bought an older 18x4 CX-9 wheel. Tire is probably too old to use, but I’ll grab a new Space Miser.

I know it’s not ideal, but will it be possible to install that on the front of a 2.5T in a pinch?

Yes, the 18X4 spare wheel will fit larger dual-piston front brake calipers which are also used on CX-9.

In fact, when I purchased my 2016 CX-5 AWD, the spec sheet posted on Mazds website at the time clearly stated CX-5 AWD gets larger T155/90D18 spare. Mazda lied to me that later I found my CX-5 AWD actually gets smaller T145/90D16 like the FWD CX-5!

Be aware a new Dunlop Space Miser MKIII T155/90D18 113M compact spare tire is pretty expensive, about $150 each!

 

[jaycal6]

Yes, the 18X4 spare wheel will fit larger dual-piston front brake calipers which are also used on CX-9.

In fact, when I purchased my 2016 CX-5 AWD, the spec sheet posted on Mazds website at the time clearly stated CX-5 AWD gets larger T155/90D18 spare. Mazda lied to me that later I found my CX-5 AWD actually gets smaller T145/90D16 like the FWD CX-5!

Be aware a new Dunlop Space Miser MKIII T155/90D18 113M compact spare tire is pretty expensive, about $150 each!

Thanks! I was hoping the CX-9 spare would work. Found a wheel and tire in good condition for $85, but just because of its age I’ll have to get a new tire. Which is surprisingly pricey, yeah, but worth it for peace of mind.

 

[yrwei52]

I don't think Mazda North America provided us with a spare that would damage the drivetrain. In particular, the manual does not say how far you could drive with the spare on. Sure, the spare is skinny and will not last for very long, but it seems that it will not damage the drivetrain as well.

If all this is correct, then it could mean that people don't need to replace all 4 tires for the sake of the drivetrain, only perhaps to keep traction identical, as slightly different diameter tires (with the difference being << 3 inches, few millimeters) will not damage the vehicle.

The difference is about 2.2 inches / 56 millimeters on overall diameter between the 18” and 16” compact spares. The overall diameter on T155/90D18 is about the same as the factory 225/55R19 and 225/65R17 road tires. When you have a T145/90D16 compact spare with 2.2” smaller on diameter than other road tires, it will have about 72 more revs / mile than the other 3. For the AWD this definitely will affect the clutch pack in rear differential greatly as it’d be trying to compensate the different rotations constantly. The ABS and TCS will get affected too as the ABS rotation sensors will detect the faster rotation on the smaller 16” compact spare, and making constant adjustment on brake force accordingly. For gen-1 CX-5 TPMS which uses ABS rotation senders, it’ll get affected too.

See the big diameter difference between 16” and 18” compact spares:

Mazda CX-5 OE Tires	Size	Overall Tire Diameter
Yokohama Geolandar G91A	225/65R17 100H	28.5"
Toyo A23	225/55R19 99V	28.9"
Yokohama Y870B	T145/90D16 106M	N/A (26.3" estimated)
Dunlop Space Miser	T155/90D18 113M	28.5"

Calculation aside, here's the spec of overall diameter from all OE tires used on Mazda CX-5 and supposed T155/90D18 spare for CX-5 AWD. Dunlop Space Miser T155/90D18 113M tire is the spare found in CX-7 AWD and 1st-gen CX-9 AWD. They're all within reasonable 0.4" tolerance on overall tire diameter.

 

[murky]

Beware Ebay listings. I purchased the correct size 17" tire first, because it was a good price at SimpleTire, like 100. Then Scoured ebay to find a used 17" Mazda spare wheel. I "won" bids 2 or 3 times in which the tire/wheel that was listed as 17" turned out to be 18" when it arrived.

The 18" wheel would have been perfectly suitable, except I already had a 17" tire. I wound up buying a brand new wheel, also at a pretty good price but over $100 instead of $40 or $50 or whatever it was for the used one.

If you want to try what I did, I'd recommend procuring your wheel first, and then getting the correct tire after confirming that both tire sizes are obtainable at a reasonable price. I'm assuming you'll want new rubber, bu tthe age of the steel wheel is immaterial. The nice thing with getting a Mazda wheel is that it will be hub centric at the correct bore diameter.

 

[Chris_Top_Her]

Look for a pre owned matching wheel probably will have a tire on it too.. saves headaches. It'll probably pay for itself after the first blowout/bent wheel

 

[Chocolate]

Ordered a replacement OE spare from a 1st gen CX-9 from LKQ. They sent me the wrong wheel, but appears to be a 17" OE Mazda replacement, bolts right on but tire size is incorrect. I got my money back, kept the wheel, and ordered the correct wheel from another CX-9. Much closer in size than the standard 26" class wheel and fits in the spare wheel well. Could have dual spares if I change out the tire on the 17" steel Mazda rim. 165/90R17 is 28.7".

 

[murky]

T165/90R-17 in nearly identical to the GT's 225/55 R19

Wheel, Spare - Mazda (9965-02-5570)

The wheel is 17x5.5" and fits mazda hub and $96 at BAM Wholesale

Mazda Wheel Disc Steel 9965-17-5570 | Auto Parts

9965-17-5570 - Wheel Disc Steel | Auto Parts

www.bamwholesaleparts.com

The 17" tire is $93 at Simple Tire.

$92.96 - Kumho (121) Original Equipment T165/90R-17 tires | Buy Kumho (121) Original Equipment tires at SimpleTire

We have your Kumho (121) Original Equipment T165/90R-17 tires at a discounted price of $92.96. Save on them today and we'll ship to an installer near you. Saving is Simple.

simpletire.com

 

[yrwei52]

T165/90R-17 in nearly identical to the GT's 225/55 R19
View attachment 222668

Wheel, Spare - Mazda (9965-02-5570)

The wheel is 17x5.5" and fits mazda hub and $96 at BAM Wholesale

Mazda Wheel Disc Steel 9965-17-5570 | Auto Parts

9965-17-5570 - Wheel Disc Steel | Auto Parts

www.bamwholesaleparts.com

The 17" tire is $93 at Simple Tire.

$92.96 - Kumho (121) Original Equipment T165/90R-17 tires | Buy Kumho (121) Original Equipment tires at SimpleTire

We have your Kumho (121) Original Equipment T165/90R-17 tires at a discounted price of $92.96. Save on them today and we'll ship to an installer near you. Saving is Simple.

simpletire.com

Mazda gives a standard 185/80R17 99M tire as the spare on CX-5 for Mexico market. This tire has the same overall diameter as 17” and 19” road tires and can be another option for those who don’t want a compact spare. The only problem is this size of the tire is very difficult to find in the US.

 

[ALafya]

"This tire should be used only for an emergency and only for a short distance.
Use the temporary spare tire only until the conventional tire is repaired, which should be as soon as possible."

"The temporary spare tire is lighter and smaller than a conventional tire, and is designed only for emergency use and should be used only for VERY short periods. Temporary spare tires should NEVER be used for long drives or extended periods. "

These sections do not appear in my manual!

For those thinking of not having a spare at all, it definitely depends on where you plan to drive your car. If only in town / never far from civilization, then maybe you can get by with roadside assistance. For those of us that occasionally drive to the boonies, where there is no cell reception for miles, you'd want something more than hiking gear to get you back home.
There, it would be unreasonable to restrict driving distance to many miles.

Before posting this thread I thought that when changing tires on an AWD vehicle, you'd want to change all four, not just two and definitely not one. One obvious reason was equal traction. However, I also thought not doing this would harm the AWD system. Keep in mind, we are talking about very few mm difference, not 3 inches.
Even if you get a different spare, which is exactly the same diameter as your full size tires, this will only be for *new* tires. Your spare is barely used, so it will be a few mm larger than your other tires at some point.

I don't know what the car will do differently when a different size tire is mounted. I guess nobody knows except Mazda. I am not convinced the rear clutch will behave differently. If anything (for a smaller wheel), it will detect 'slippage' in the rear and divert more power to the front, by not engaging the rear clutch. However, it may not do that at all since it can tell that the diameter of the wheel changed when you got a flat and can therefore prefer to send power to the front only.
This is all speculation and could be different from vehicle to vehicle.
We can't even tell if the warning above is for safety, drivetrain reasons, expected longevity of the tire, or some combination of these.

 

[sm1ke]

These sections do not appear in my manual!

I pulled those quotes directly from the 2014 Mazda CX-5 Owner's Manual found on the Mazda USA website. Specifically pages 7-5 and 7-6.

For those thinking of not having a spare at all, it definitely depends on where you plan to drive your car. If only in town / never far from civilization, then maybe you can get by with roadside assistance. For those of us that occasionally drive to the boonies, where there is no cell reception for miles, you'd want something more than hiking gear to get you back home.
There, it would be unreasonable to restrict driving distance to many miles.

On page 7-6 of the manual, it also mentions that the spare tire should not be mounted on the front axle. Mazda's AWD system is primarily a FWD system that sends power to the rear wheels when traction loss is detected. If you put the spare on the rear, and reduce your speed appropriately, you should be able to drive as far as you need to with minimal stress to the drivetrain, since the system is primarily concerned with front wheel traction. I suspect that this is why they don't specify a maximum driving distance when the spare is in use.

Before posting this thread I thought that when changing tires on an AWD vehicle, you'd want to change all four, not just two and definitely not one. One obvious reason was equal traction. However, I also thought not doing this would harm the AWD system. Keep in mind, we are talking about very few mm difference, not 3 inches.
Even if you get a different spare, which is exactly the same diameter as your full size tires, this will only be for *new* tires. Your spare is barely used, so it will be a few mm larger than your other tires at some point.

I don't know what the car will do differently when a different size tire is mounted. I guess nobody knows except Mazda. I am not convinced the rear clutch will behave differently. If anything (for a smaller wheel), it will detect 'slippage' in the rear and divert more power to the front, by not engaging the rear clutch. However, it may not do that at all since it can tell that the diameter of the wheel changed when you got a flat and can therefore prefer to send power to the front only.
This is all speculation and could be different from vehicle to vehicle.

We can't even tell if the warning above is for safety, drivetrain reasons, expected longevity of the tire, or some combination of these.

A few mm is probably something that the AWD system can account for. Additionally, if you had to, you could buy a new tire and have the rubber shaved down so that the diameter matches that of the other wheels.

The bolded portion is, IMO, the most important part of what you said. As individual owners we are free to do whatever we want, however we want to do it. Personally, doing something based on speculation, while the manufacturer recommends the opposite, is a bad idea.

Speaking for myself, I would much rather err on the side of caution, than rely on speculation.

 

[ALafya]

The main take-away for me is that changing all 4 tires is equally encouraged for both FWD and AWD and that, at least in the rear for the CX-5, even a huge diameter difference will not cause drivetrain damage for 10s of miles and possibly few 100s of miles.
Regarding speed, I believe this is for safety, as any turn or maneuver with such a different wheel could cause loss of control and an accident. Of course, the tire's speed rating should also be respected, M for 81 MPH.

 

[murky]

The main take-away for me is that changing all 4 tires is equally encouraged for both FWD and AWD and that, at least in the rear for the CX-5, even a huge diameter difference will not cause drivetrain damage for 10s of miles and possibly few 100s of miles.
Regarding speed, I believe this is for safety, as any turn or maneuver with such a different wheel could cause loss of control and an accident. Of course, the tire's speed rating should also be respected, M for 81 MPH.

I'd change that to ..."will not cause immediately apparent, catastrophic drivetrain damage"...

Put another way, I guarantee it wasn't an engineer who proposed putting that under-height tire in the spare well of the AWD CX-5.

 

[yrwei52]

⋯ A few mm is probably something that the AWD system can account for. Additionally, if you had to, you could buy a new tire and have the rubber shaved down so that the diameter matches that of the other wheels.

The tread depth of a new tire is 10/32” (~ 8 mm). So you want to shave some mm (< 8 mm) off a new tire to match the other 3 partially worn tires, but you’re not worried about using a factory spare which is 56 mm too small on total diameter than other 3 road tires on an AWD CX-5 which will upset AWD and TCS/ABS systems???