Differenting tire sizes

This morning I found my car with a flat on the front driver side of my 2002 Kia Mini Van. I pulled out the spare which is a full size tire except it is not the same size as the other three tires on my car. My other 3 are 215 and the spare is 205. The AAA guy was not to keen on putting on the spare so I could drive to work and back today (30 miles) until I can get to the tire place tomorrow to solve the problem. I am not sure what the spare is 205 and not 215, all the other size things are the same as the others. I did finally get hiim to put the spare on and I drove to work without any issue. I couldn’t tell any difference how the car drove. Am I doing any damage to the car driving with this tire issue?

I assume your car is front-wheel-drive. If so, putting it on the back temporarily would be okay, but I’d be nervous about putting it on the front.

Running the 205 for a spare doesn’t hurt anything. Look on the door plate and see what size tires are called for. Even if it is 205, running 215s won’t hurt anything unless they rub. Either get the flat repaired or a new tire.

The 205 is just 10mm narrower than the 215. As noted, it won’t matter. I’m assuming the other specs are the same between the two kinds of tires?

the specks on the door call for 215 which is what the four tires that are on the van are. The spare is exactly the same except for it being 205 and not 215, the 70R is the same. It seemed to drive just fine to work, I just counldn’t get over how the AAA guy was going on how dangerous it was and was considering not installing it and wanting to just inflate the flat and follow me to a tire place to get repaired. I informed him I didn’t have the time to do that and that isn’t a “donut” spare smaller then the other 3?

He’s worried about something going wrong and being sued, I can understand that. I would replace it with a 215 at your earliest convenience, I don’t like mismatched front tires, even if it seems to drive OK. And yes, ‘donut’ spare can be smaller, but are for temporary use.

You can potentially burn up a differential on the driving wheels(front) if you have mismatched tires.

I don’t plan on driving with the mismatch for more then a day or so, should I be okay for not screwing anything up? Would it be better to put it on the back until I can get a new tire?

Yes, short term no problem. Since I’m pretty sure it’s just a simple differential, it won’t ‘burn up’, just have slightly more wear.

The tire size issue relevant to differentials 1) generally doesn’t apply to FWD vehicles (within reason), and 2) is about tire diameter. The 205/215 difference is about width. Its unlikely that your spare is precisely the same diameter as the other tires, but the size difference is not a diameter difference. Its not really going to matter much - especially if the spare is on the back.

right now it’s on the driver front. AAA inflated the flat tire and it’s in the back of my van, I will check it tommorow morning to see if it’s flat again. If not I will just put it back on. Not sure why it went flat if no hole, I did hit a small/medium size pot hole last night and it did get very cold here so not sure if that was the reason.

Exactly what does the 205 70R 15 mean?

Here’s the secret decoder ring as it relates to tires:

cigroller wrote:
The tire size issue relevant to differentials 1) generally doesn’t apply to FWD vehicles (within reason), and 2) is about tire diameter. The 205/215 difference is about width.

The diameter is different, by about 2% if I’ve done the math correctly.

cig is absolutely right. It’s more about the diameter difference then the width. Though width is indirectly an indicator of diameter comparison on tires (times aspect ratio) with the same rim size, that’s not true if the rim sizes are different. We assume that the spare will have a larger diameter rim to go with the smaller width if they are to be nearly equal diameter. If they are not and it’s on the rear, no problem except for handling. If placed on the front, head the limited number of miles and speed as recommended in the owners manual, regardless.

This spare is on the same sized rim as the other tires, and is the same aspect ratio, so it’s slightly smaller. Space saver ‘donuts’ are MUCH smaller in width than the regular tires, not just 10 mm.

bertrand, it could go flat from hitting a hole if you bent or damaged the rim. It could now be leaking air from the rim and may require you to replace the wheel itself

You need to learn a bit more about tire sizes in order to understand why the roadside assistance guy didn’t want to put a different size tire on your car.

So lets look at the size info, and decipher what it means so that you can understand it.


width/aspect ratio - wheel diameter

So that first number, the 215, means how wide the tire is.
In this case, it is 215 mm’s wide.

The second number, 70, is the aspect ratio.
To put it simply, this is a percentage of how wide the tire is, and this tells you how tall the tire is from the rim to the tread of the tire. 70% of 215mm is 150.5 mm’s.

And the third number is the wheel diameter.
In this case, its a 15 inch wheel.

Now, your spare tire which you have implied is a 205/70R-15, has a width of 205, and a sidewall height of only 143.5 mm’s. This means that the spare tire is shorter than the majority of the tires on your vehicle.

Putting a shorter tire on one corner of the car means that its going to be spinning faster than all the other tires, will cause more stress on the wheel bearings, and possibly the suspension components, and potentially can damage a differential if you put it on a driven wheel, under certain circumstances.

On some vehicles, this is a really expensive thing to do, while on other vehicles, it might not cause any damage at all, no matter how long it is installed in place. Vehicles that have limited slip differentials usually warn never to use the spare tire on the driven wheels, because of the potential for this damage. They recommend mounting the spare tire on the non-driven wheel, and switch that tire to the flat tire location. Mostly, this falls on rear wheel drive sports cars.

You should have that tire replaced with a 215/70R-15 so that you don’t ever have to deal with this situation again in the future.


I have the flattened tire in the back of my van, since it went flat overnight 8:30pm on Thursday found flat at 10am on friday, my thought is if it is still full of air come Sat morning everything is okay, if not we know there is a leak. I have hit bigger and badder pot holes before and done no damage, so when I hit the one last night (which I knew was there and just forgot to swerve) wasn’t that bad, but we will have to wait and see.

As for the spare it’s a “General” tire on a normal rim. I think the place where I bought the van ( a used car place that also did repairs and tires) removed all the tires including the spare and put on 4 lesser tires because all the tires are not from the same make, and I wonder if by accident they grabbed a 205 when it came time to replace the spare not a 215.

Once I get the regular tire fixed I am going to and buy a used correct sized tire for the spare.

Question, isn’t the tire I have on now at 205 not shorter then a 215 but just not as wide?