Scale is the size of the model as compared to the size of the real car. Ergo, a 1/24th scale model is 1/24th the size of the real car it represents. Generally, 1/24th scale models are too big to mount on the 1/28th scale XMods chassis.
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-- It is by will alone I set my mind in motion...
Some of them work, though. I have one on right now, and it fits. The key is to find a model from a car that is short in real life. In 1/28th scale for every 28 inches (or whatever measurement) on the real thing, there is 1 inch on the model.