Hmm, I just noticed something about the Fleetrunner belt. It has a wider top by 1mm and has 2 degree different angle compared to the Gates XL and the cheap Masterpro. The top probably wouldn't matter much, but what about the different angle? I might have it backwards, but a Fleetrunner 38 degree angle would make it have less contact with the pulley's side walls, vs 36 degree. I can't find the angle spec on the pulleys to know what they are.
I may be able to help you understand that variance in the specification. I replaced my alternator belt back in 2015 with a special Gates high performance green stripe(?) belt which was hard to find anywhere thereafter. It may have been the precursor to the Fleetrunner belt. (By the way, the Gates website has
extensive documents and specifications for their numerous belts).
The belt that I used was oversized, much as you describe for the Fleetrunner. It is purposely designed that way so that: When first installed it rides high in the pulley V, appears to be a little too wide as well, BUT after running it for awhile (not very long, like maybe hours or a few days or a it longer depending upon how much you drive) it seats down into the pulley for a very good grip and fit. That is what it is supposed to do and that's why it's designed that way. I found that info at the Gates website, in a belts specifications PDF.
I can't look for the package it came in tonight but I DO HAVE IT in a box with spare belts and I can find it tomorrow in the daytime and post here which belt it is.
Also I found that this Gates belt flopped around a bit less than the previous belt that was on it. It also straightens out as soon as the RPM goes up. Most belts should do that and it's probably not possible or necessary to get rid of that flapping on that very long upper span of the alternator V belt.
I swapped out the belts and other components that didn't really need to be changed yet because I just want all new and dependable parts under the hood and elsewhere. I want to get all of that done while it is convenient to do before I take this truck on a long haul, towing a trailer over some mountain ranges etc. and I DON'T want to be stuck on the side of the road in a remote area (or anywhere) with a full bed and full trailer loaded with valuable possessions due to some replaceable part that failed.
Lastly, I bought some of those
'close-out' low priced belts from Rock Auto Parts for temporary spare belts because I couldn't afford to buy expensive spare belts at the time. I want to mention that some of these 'close out' belts could be quite old, or they could be fairly new. If they are quite old, they probably won't last as long as a newer belt would, simply due to ageing of the rubber. That's why I am considering them as temporary low cost replacements.
Gates and probably some other big name rubber companies also make OEM labelled belts for auto manufacturers. I'm quite sure that I read somewhere that Motorcraft belts were/are made by Gates Rubber Co.
That should come as no surprise if you know much about Gates Rubber Company and their
excellent long history in the research, design and development of rubber belts. I also believe that other big name rubber companies can make
very good belts but have several reasons for selecting Gates as my top choice but I don't have the time or convenience to explain why.