"Needle float" problems usually lie in the cable-Sometimes adding speedo lube/graphite will lessen or solve, sometimes not. I have a Chevy G30 that had the dreaded speedo float accompanied by an obnoxious "tic, tick, tick. The cable itself was hanging up inside the cable sleeve as it spun. Rather than mess around with the lube or having to be right back fooling with the same issue 2 months down the road, just ordered a new one from Advanced Auto. (Was pleasantly surprised to learn a '85 G30 speedo cable could still be had!) Works great now. Done & done.
Generally speaking, yea, oversized tires will throw off your actual speed indication but creating the 'float" isn't commonly associated. Indicating the speed of the vehicle involves the trans, the speedometer and the cable itself. No computers, no Apps etc. Ya gotta remember, its an EB, not something NASA built. LOL
Tell ya how I determined it was my cable; I detached the cable at the trans. Then I attached a battery powered drill to the cable end. While watching the speedometer, I slowly spun the cable with the drill. The "float" was still there (at low speed) so it eliminated the trans as the problem. Increasing the drill speed made the "float" go away. At that point, I leaned toward the cable as the cause so I replaced it while hoping for the best. (Not too hard to replace because I already had the other end disconnected) Whew!, as it turned out, like I said-it was the cable!
I'd start by /servicing/replacing the cable. It's the easiest and least complicated place to start...