Ersatz Tires

Flat tires aren't as common as they used to be (due to better materials and construction), but they still happen at the worst times. Since spare tires don't get used very often they tend to get ignored, which means that there's a good possibility that your spare is going to be flat when you need it.

The “temporary” spare, AKA “the doughnut” that comes with most cars, is there to get you to the nearest repair shop. Most of them have a very limited life, usually under 100 miles. Doughnuts are also smaller and narrower than the standard tire, so they will affect the handling of your car when in use and put a lot of stress on the differentials of vehicles because of the difference in size (smaller tires have to spin faster to cover the same distance).

Since 2009, some car makers have decided to save even more weight and space by deleting even the doughnut spare, replacing it with a “repair kit” that consists of a can of sealant and a compressor. The repair kits are useless if the tire has anything other than a small hole in the tread, but they're cheap and save weight (and therefore gas). Basically, they've started shipping cars with a can of Fix-a-Flat instead of a spare tire.

So what do you do if you have a flat tire and your spare is dead? 

Or what if you have two flat tires at the same time (Been there and one that - got forced into a curb that took out both tires on one side of my truck)? 

1. Look around and see if you can borrow a spare from someone. This may sound strange to urban people, but rural folks do tend to help each other and there's always a chance that you'll be able to find something that will fit your car. 
2. If you're in a SHTF situation, salvaging a wheel and tire from an abandoned car might be your only option to get mobile again.  Unfortunately, not all tires are created equal; fortunately, this article will tell you what need to look for:

Tire Size

For emergency use, this is less important than you'd expect. Since the typical doughnut spare is a lot smaller than a regular tire, you can use any tire that will fit inside your wheel-well, and running smaller (a 14-inch tire replacing a 16-inch tire) won't be much different than using a doughnut.

Going larger or wider means you'll need to check the clearance on your fenders and steering components; you don't want a tire rubbing on anything. Here'show to read the information on the side of a tire.

Watch the weight rating! You'll want something that is rated for at least as much as the tire you're replacing, especially on trailers.

Number and Spacing of Lugnuts

Most standard wheels will have 4, 5, 6 or 8 holes for the lugnuts. 4- and 5-hole wheels are common on cars, 6- and 8-hole wheels are more often found on trucks and SUVs. Ford did make a 7-hole wheel for a few years on their F250 pickups, and there are some trailers and ATVs that use a 3-hole wheel, but those are the exceptions. Obviously, you're going to need something with the same number of holes as your regular wheels.

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Spacing is a bit tricky, but there is a standard way of measuring it. The picture to the left shows how to measure your bolt pattern. This needs to be an exact match, but as long as you're working with two vehicles that are similar in size and age, you should be able to find a suitable wheel.

Center Hole Size
The hole in the center of the wheel is for the wheel hub, and there are two general types of wheels:

Hub-centric: where the weight of the vehicle is borne by the hub of the wheel. This style is common on cars, and these normally have a raised lip on the brake disk/drum around the hub that the wheel slides over. This needs to be an exact fit since the lug bolts are probably not going to be strong enough to carry the weight, and changing sheared lugs is a royal pain.

Lug-centric: where the weight of the vehicle is borne by the lugnuts. This style is more common on trucks and larger SUVs with 8-hole wheels. The raised lip is absent, and the hub hole diameter is less important. As long as the hole is big enough for the hub to go through, it'll work. If you have a 4WD with locking hubs, you'll know how big some of these holes can be.

Depth of Wheel

Unless you're playing with custom wheels this isn't much of a problem, but finding a donor wheel with the proper depth to give you room for the brake parts is important. Here'sa good link that explains the depth measurements. In a SHTF situation, I'd try flipping the wheel around and putting it on backwards (with the outside of the rim towards the vehicle) just to get moving again.

If money's tight and/or you really need to get out of Dodge
I know of at least one young man who buys temporary spares from a local junkyard for next to nothing and runs them on all four wheels of his compact car. He has to change them every week or so, and they are horrible for traction, but they get him to work and back. He's saving for a better car and doesn't want to dump $400 into tires for something he's going to be trading off in a few months.

Where there's a will, there's a way.