I only get 3.5" on the seat stays up to just over halfway to the tire...at where the bulgiest part of my tire I'm down to 2.95". Chain stays are 3.25" until just shy of the rim, and then start their curve in.
View attachment 114620
This pic is with the chain off and the wheel pulled all the way back in the drops...max clearance
. Distance from knobby to stay in this pic is just under .12"...with the axle all the way back.
My tires are low...and I don't have an air gauge here. Back measured 2.25"…front 2.24". (aired up the front some, now 2.27"). So, for every .1" you add in tire width, you add .05" to the tire cross section radius. Going to a 2.45" (+.2" to my measured 2.25") will eat up .1" of my .12" chain stay clearance, leaving .02"... which isn't much
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It could be that there were changes over the years of the HD...you may have more room than me. Some ideas to maximize tire size would be to lengthen the dropout slots...or, some judicious tube crushing like Carl mentioned...or, more radically, remove the kickstand mount and reshape the chainstays.
