Tonight I did a bit of tire research. The car has brand new Michelin Pilot Sport 4's on it, with a very slight oversize (205mm vs 195mm, +1.7% diameter). I have not yet tried to figure out of the odometer reads high or low. This tire size actually sits right in the middle of the two MX-5 trims - some models have 17" wheels with tires another 0.8% larger.
Anyhow, I discovered a site that did comparisons of a lot of the tires I have interest in:
https://alltyretests.com/michelin-en...s-test-review/
https://alltyretests.com/michelin-pilot-sport-4/
In particular:
One thing that surprised me was how well the Vredestein Sportracs fared in their rolling resistance test. These are what I put on the front of my muscle car engine swapped Insight, and I had been surprised to find they didn't hurt fuel economy much compared with the OEM low rolling resistance tires.
Another thing, the hyper aggressive summer tires on the car really aren't
that bad.
So, plugging those values into Ecomodder's calculator (and making some educated assumptions), I got the following:
Swapping the coefficient of rolling resistance value to that of Michelin Energy Savers appears to be worth approximately (and optimistically) 2mpg at highway speeds.