I have heard of doing this on Harley whitewalled tires too. I have BFG's to and they are yellowed. Guess Ill have to try this.Put a little lacquer thinner on a rag. Simply wipe the letters with the damp rag. Zero effort. If you use tire shine, or armor all it turns them brown/yellow fast.
The tires everyone is talking about are new production from the last few years and once cleaned up it takes just a very short time before they yellow (actually brown) again. It is very possible size makes a difference as BFG retires some sizes and Coker is licensed to make the retired sizes.I've used BFG solid white letter tires for decades, never had that problem. I use Westly's Bleche White with a stiff bristle brush to clean them, makes them bright white. I never use tire dressing.