You need to get into the details of "size" to answer this. The three aspects of size that are important are the tire diameter, tire width, and tire load (weight) carrying capacity.
I'm running 11R22.5's as replacements for my rear 295/80R22.5 and they are almost an exact match for the tire diameter but somewhat less in load carrying capacity. The 12R22.5 is a little bit wider and taller, with more load capacity. It more closely matches the 295/80 for load but the speedometer/odometer would be inaccurate because of the diameter difference. The 295/80 makes more revolutions per mile than a 12R. The 12R is also wider than a 295/80, which is important consideration for rear duals.
Since you would be going slightly down in size with the 295/80, your speed and odo would read slightly low.Dual spacing should be fine because the 295/80 is a bit narrower. Just make sure the max load capacity is sufficient for your front axle (only the front will need checking - you will have plenty for the rear and tag). Most 12R and 295/80 have the same 7830 lb max load rating, but double check that before buying.
2004 American Tradition; 2014 Buick LaCRosse
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