If even the EPA for CAFE measures doesn’t attribute changes to a single variable while using multi-million dollar equipment to measure fuel economy, what makes you think it can be done with a pen and paper in an uncontrolled environment? Especially when you can’t even use the odometer for a point of distance traveled? You even admitted as much- the height difference in tire size should be greater than 3%, but you claim rollout is less than 2%. Which one is right? Or are they both wrong? Either way, your calculations are making ASSUMPTIONS all the way around, trying to fill in data where no reliable source exists. How do you account for the energy between summer and winter gas? Air temperature and density? Humidity? The list of impacting factors goes on nearly forever.
If your data held true merit, you can nearly guarantee every vehicle manufacturer would be jettisoning wider tires and also slashing their budgets on all other R&D since you found the holy grail of efficiency improvements.
You may have seen some improvement, sure. But you can’t pin any enumerated % of the gain on tires, just like I can’t pin it on oil.
I don't pretend to have a full understanding of how CAFE works, but if my limited understanding is correct, among other things the manufacturers are often chasing tenths or hundredths of a point gains, especially in higher volume models. When I first got my MKZ(I doubt a particularly high volume model, especially compared to its platform mate the Fusion), it was still on factory tires. They were Michelin MXM4s, a tire that was a known entity to me I thought, but I was never happy with the ride quality of them. Granted part of it could have been that they were closing in on 8 years old(yes the first thing I did was throw new tires on it, even though there was a decent bit of tread left). Still, though, I had been around this car and driven it occasionally since it was new. My grandfather bought it new, racked up about 15,000 miles, my dad bought it from him in 2014 mostly to keep him from driving it, and I bought it from my dad in 2017 with 25K miles when it was finally time to replace my much-beloved but aging Lincoln LS(what I wouldn't give to have another of those). I'd run MXM4s on the LS(although I bought it post-factory tire, which was also an MXM4) and even with its "sportier" suspension I thought it rode better on them than the MKZ ever did.
When I took the MKZ to a trusted tire shop I dealt with for years, I mentioned that I'd love to put another set of MXM4s if they could ride like the ones I'd had in the past. They pointed out to me the "Energy" and "Low Rolling Resistance" labels on the factory tires, and said they could order a few different versions of that tire and were sure I'd be happy if they ordered the right one. Ultimately, that trip I put Bridgestone Potenzas on, another tire I knew and trusted, mostly because of the buy 3 special were running then on Bridgestones. I have the MXM4s on it now and have been super happy with them, but am shopping for my next set and we'll see where I end up.
Also, it's worth mentioning that the MKZ was supposedly the replacement for the LS, although about the only thing they really have in common is the general size. The MKZ came with a 245/45 and the LS a 235/50, so despite stricter CAFE standards in 2010 as compared to 2004 they actually widened the tire, although of course the FWD V6 MKZ is a whole different beast in gas mileage than the RWD V8 LS.
My MG was well before CAFE standards, of course, but it's a place for me to make an interesting observation. In 1970, the car shipped with a 165HR14, which is 165/82R14 in modern terms. At some point earlier in production, the width was 155. That size is basically unavailable now, short of specialty suppliers. Lucas Classic Tire at least use to sell a Pirelli 165HR15 essentially identical to what would have been on the car new. For a while I ran 185/70s since that's an easy size to find and is about the closest to the original in diameter, although now I have 175/75s. I've driven other MGs on 165HR14 Vredestain Sprints(I'd consider those, and they are a little easier to find and a little less expensive than the Pirelli), quite a few on 175 and 185s and a couple on 195s. One of the things I've noticed was that steering effort definitely increases on wider tires(no power steering on these) and overall the skinnier tires just make the car feel a lot more "lively." I did once drive a GT I was seriously considering purchasing that somehow or another had ended up on 215s. That's way too wide of a tire for the RoStyle steel wheels they were on, and they actually did visibly "balloon" on the tread. Aside from scrubbing in sharp turns, it was almost scary to drive, and the steering was so light that if you had driven other MGBs, you'd have been forgiven for thinking it had power steering somehow or another. If I had bought it, I'd have driven it directly to the tire shop...
My point in all of this-I think of the IS250 as being a sports sedan, although maybe that classification is mistaken. The OP has already noted decreased steering effort on the skinnier tires, and I suspect many buyers might have appreciated how a high end skinnier tire would have handled on the car. If it could have made the car more fun to drive and given a fuel economy boost, it seems they would have considered it both an engineering and sales/marketing win-win and would have done it.
