Can you advise the regulation(s) with respect to tire size on a passenger vehicle?
I am considering using existing winter tires on a newly acquired vehicle but the existing tires are slightly larger than the new vehicle tire size. The change would result in the larger tires being approx. 3.5% larger circumference than the new vehicle tires.
What is the law in this regard?
The Motor Vehicle Act Regulations and the Superintendent's Standards (vehicle inspection manual) are mute on this. They require good tread, no defects, manufacturer's markings, same size tires on each axle and the like, but don't say anything about a specific size of tire for a vehicle.
Your owner's manual does though and this is important, particularly on newer vehicles. 3.5% isn't much, but I could not say how this might affect your speedometer reading and things like anti-lock brakes or vehicle stability controls. If your vehicle has these things, I would consult the appropriate car dealership for your make and model, then follow their advice.
Sounds to me like you've done your research, but just to try and be generally informative, I'll add this.
Tire sizing is a weird combination of imperial, metric, and mathematical factors.
As an example, if the tire is specified as a 215/60R16, what that means is that this Radial tire will fit on a wheel rim with a 16" diameter, and will have a 21.5 centimeter width. Meanwhile, the sidewall measurement would be 60% of the tire width - in this case (reaches for calculator) that would be 12,9 cm. This is known as the aspect ratio.
Various combinations of these may provide a better footprint, without compromising speedometer accuracy. Most major manufacturers have a 'basic' sort of wheel and tire that will come with their economy model, while the 'deluxe' version will almost certainly be one or even two inches greater in wheel rim diameter, with the aspect ratio adjusted so that it still fits properly and safely on the same car - 215/60/16, 225/50/17, 225/45/18, 225/40/19 for instance. The ever increasing wheel diameter is offset by the lowered aspect ratio, will no doubt cost you more, and quite possibly pound your spine into submission on rough roads - or speed bumps. But it can provide superb grip (reaches for 5-point harness) which is why you won't see bicycle tires on a Ferrari ...
What the vehicle owner should be aware of, when using a different size of wheel/tire than the original specification, is the possibility of the exterior of the tire making contact with the vehicle, which ain't supposed to happen. Maybe not too serious if it's the tread (which is very tough) against the inside of the wheel well when you go over speed bumps, maybe quite serious if it's the sidewall (which is not so tough, as it needs to be pliable) hitting the hard parts when the front wheels are turned sharply.
There's actually a nifty website, Wheel-Size.com, that can help a vehicle owner figure out the viable possibilities.