Objects that are likely to suffer a great deal of damage during the course of play should have a Durable stat 0 to 20 points. In addition to that, they have Wear & Tear. Wear is like Sweat for objects, and Tear is like Blood for objects. The Durable stat indicates how much damage can be taken to Wear when the object is hit in combat (like the Tough stat indicates how much damage you can take to Sweat). At 20 Wear, further Wear carries over to Tear. At 20 Tear, the object is completely broken beyond repair (well, except by high magic).
Wear can be reduced by ordinary maintenance. So, you can reduce the Wear on your tires by airing them up, or patching them, etc. This reduces it by half, like Sweat reduces when you get a chance to rest. Tear can only be fixed by replacing the object or parts of it. Using jury-rigged parts for this is fine, but won't work as well as the real thing (i.e., half as many points restored).
In the case of vehicles, it might be a good idea to track Wear & Tear on the tires, the engine, and the body separately as it becomes necessary.
Any comments would be appreciated! Post 'em here or email me: marksman45 [at] gmail.com