Could a smaller spare damage your differential over long distances?

[Big Kahuna]

Got my first flat tire and my “full size spare” was a 215/60R17 and 27.2 inches compared to my stock 225/65 pirelli tires at 28.5.

I read through the manual and it seems like the full size spare isn’t a 50mph max tire it’s a 70mph max.

So my question - on a long road trip if you use the full size spare for say 500 miles but don’t go over 70 could you cause any damage?

Now many guys on here have gone to a 245/70 which is a full 2” larger than the full size spare which made me think in their case it might almost be acting like a donut tire and they’d need to go max 50mph to prevent damage?

Im moving to 235/70 tires and am thinking I’ll put a 235/65 AT for a spare if a 235/70 won’t fit as it would be 29.1” with 30” tires vs the 27.2 current spare.

Just wondering peoples thoughts on the tire size differences for the spare tire and risks.

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[Rivers90]

Is your truck all wheel drive? If not all wheel drive and this was on the back of the truck on the non powered rear I don't think it would matter too much.

But even if it is fine, you will have reduced traction, reduced braking, and other performance hits.
I could see the computer not liking one wheel to be turning at a different speed.

The tires are close in size, so it should work but I would still stick to what the manual says about the limits of driving on a spare tire of a different size.

On cars with a true limited slip differential this would make for all kinds of problems and you could damage something, but we don't have that.

I did get a matching spare just to avoid these problems and limits. All my tires are 235/65r17

I think there are posts from people who went with bigger tires also going to a bigger spare.

Depending on if you have the hitch or not size maybe limited. Some people have taken the air out of the space to fir a bigger one.

 

[Big Kahuna]

Is your truck all wheel drive? If not all wheel drive and this was on the back of the truck on the non powered rear I don't think it would matter too much.

But even if it is fine, you will have reduced traction, reduced braking, and other performance hits.
I could see the computer not liking one wheel to be turning at a different speed.

The tires are close in size, so it should work but I would still stick to what the manual says about the limits of driving on a spare tire of a different size.

On cars with a true limited slip differential this would make for all kinds of problems and you could damage something, but we don't have that.

Yes I have the AWD FX4 4K. I think the 0.9” difference I’m planning on should be fine at 70 mph but what about those guys with 245/70 30.6” tires and a 27.2” spare? It seems like they might need to keep it under 50mph with that kind of mismatch right?

The donut spare is 155/70D17 and 25.5” tall which is a crazy mismatch even for the stock tire. For the guys that added the 245/70s if they have a donut and not the full size that seems super dangerous.

There is something in the manual about the AWD shutting off when you use a spare but no info on how it does that or what causes it to know. Guessing wheel speed? So if you put the donut on the front will it be smart enough to switch to rear wheel drive?

 

[Rivers90]

Little mini donut spares have been used for many years and they have been fine as long as you stay within its limits, I know cars even in the 1980's came with them, so there has been lots of time for the bugs to be worked out. Years ago a car magazine tested the mini spares to there limits and they did better then I though they would. I even read a story where a car had a mini spare on it and it did better in the 0-50 mph test compared to one with factory size. Something about lower gearing?

But my choice is for a full size spare.

 

[Gray Goose]

Consider this question: Is it really worth risking thousands of dollars in drivetrain repairs to save a much smaller amount to have matching tires all around? Personally I wouldn't risk it for more than a few miles to get to a tire shop.
Additional information:
https://auto.howstuffworks.com/replacing-tires.htm
https://oklahomatechnicalcollege.com/automotive-technology/tires/

 

[HPR]

The manual states to put the different size tire on the rear. The truck senses the difference via amount of rotations of each wheel and will not engage the rear wheels. The front wheels are the primary drive wheels.

 

[Gray Goose]

The manual states to put the different size tire on the rear. The truck senses the difference via amount of rotations of each wheel and will not engage the rear wheels. The front wheels are the primary drive wheels.

Good to know it reverts to FWD in case of mismatched rear tires.
Patiently awaiting my manual and the truck that comes with it.

 

[commadorebob]

I like to keep the wheel power display up on my truck. My AWD mostly only engages the rear wheels at low speeds or while climbing a steep grade. Otherwise, it behaves as a FWD.

 

[RedRider]

If you understand how a differential works, you will understand why you do not want the left and right wheels operating at different speeds for an extended period. I don't believe that any manufacturer puts bearings into the spider gears, so that's where the unacceptable wear will occur. Also remember that this wear will accumulate with each time that you use the smaller tire. At what point that wear gets to be an unacceptable runout limit is just a guess, but I'd adhere to the 50mph/70mi that every donut manufacturer recommends in order to avoid that super expensive damage.

 

[AutobahnSHO]

I would NOT go 500miles (presuming highway) on a spare tire.

 

[Discount Tire]

Over the years I have told my customers that a temporary tire is just that...temporary. It is designed to get you out of harms way and to the closest repair facility. Follow that rule of thumb and you will be ok... Oh....and don't try to break any land speed records while driving on your spare!

 

[Big Kahuna]

Consider this question: Is it really worth risking thousands of dollars in drivetrain repairs to save a much smaller amount to have matching tires all around? Personally I wouldn't risk it for more than a few miles to get to a tire shop.
Additional information:
https://auto.howstuffworks.com/replacing-tires.htm
https://oklahomatechnicalcollege.com/automotive-technology/tires/

Absolutely, but for those going to 245/70 a full size spare might not be an option. That’s why I’m thinking people should try to go to a 235/65 or 235/70 (if it will fit) even if they would only be driving a shorter distance.
In my state on the east side there can be 50-100 miles before a tire shop depending on where you are and if it’s late on a Sunday evening you’re screwed.

So my thinking is that if you can’t go to a full size spare you should at least go as big as possible for the spare in case that bad scenario does happen. A 1” difference seems a heck of a lot better than a 2.5” or 3” difference if you do have to go 50-100 miles.

If indeed you put the spare on the rear and the car automatically switches to FWD than that’s awesome but even then having less of a size difference is still probably better for the car.

 

[GPSMan]

Don't sweat it.

Did Ford provide a jack stand in your behind the seat kit? They didn't in mine.

THEREFORE You are by SOP (that's Standard Operating Procedure) to put the spare into the position that is flat.

You have a 50/50 chance it will be a rear tire.

If a front goes flat, put the spare there, reduce speed, and change it as soon as practical.

No manufacturer wants you to change two wheels on the side of the road. Any road.

In your home or professional garage, knock yourself out.

 

[Soberjoe]

Simply for information purposes: 2022 EB w/4K and FX4 package - a 235/65R/17 Falken WildPeak is my full sized spare on a 17" steelie.
I had to deflate the tire by about 10lbs and make sure that the teeth on the rear-end collision safety device are BETWEEN the treads, and yes it is still a tight fit, but its in there.
I bought a $39.99 air pump at Canadian Tire that lives in the rear-seat storage cubby.
So my spare tire is exactly like my other 4 tires.

Again I am only posting this so that those of you who have 235/65/17" are aware that a fifth identical tire will fit as your spare.

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