If Mom had been home, she'd have told you both to put the butter in a clean plastic bag first, unsealed so it won't pop. That way it could have been salvageable, and your tire wouldn't be greasy.
Depends on the goal of the experiment. If the only aim is to determine the sqishability of the butter, then a plastic bag would be acceptable as it would provide no meaningful resistance to the tire. However, if one wishes to determine the precise nature of the butter's squish, then many more experiments need to be made, both to establish a control and to analyze additional squish conditions (butter temperature, wrapper on/ off, use of plastic bag, etc.)