Needing new tires

It's not an argument. The manufacturer's specs are the manufacturer's specs, that's just a fact. If you choose to ignore those specs in this particular case, you will lose performance. That's not always true with manufacturer's recommendations in general, but it is true in this instance. Most autoX and road race types won't even run the recommended minimum width rim for a given tire as even that can produce a noticeable loss of performance compared to running the maximum recommended width rim. The fact that none of you apparently drive your cars enough, or hard enough, to notice that loss of performance doesn't mean it isn't happening. At 13 years your tires should be replaced just because of age, treadwear is almost irrelevant with that little mileage. Some car companies recommend changing your tires regardless of mileage after 6 years, and most tire manufacturer's put the top end of tire life for safe use at 10 years. How Old - and Dangerous - Are Your Tires?

With a Dart/Scamp the widest tire you're going to fit easily is a 245. If that's with an 8.25 they're narrower than 8 3/4's, so, you'd be looking at a 15x7 with about 4.25" of backspace to make that work. Some folks have made 255's fit, but that usually requires quarter lip modifications or a very stiff rear suspension to keep from rubbing during cornering or with weight in the back of the car.