Well since both regen and mileage are dependent on gas pedal pressure, it's hard to measure the exact difference since any any given time either regen is on or it's off, I can never toggle it while keeping everything else constant. But when I've driven on the highway without regen, I definitely appear to get higher mileage than with it on, and that's logical, isn't it? I can definitely feel when regeneration is on, even a little. It's alot more enjoyable when the car just glides.

But guess what? I figured out a way around it using 'S' mode! I'll post more about it when I have a chance to collect more data, but with S it's easy to accelerate using just a high-revving ICE and little to no assist. I was able to drive on the highway at 80 miles per hour with a completely full battery; when I got to the exit, the friction brakes were mostly used with barely any regeneration since the battery was full.

Unfortunately I don't have very useful MPG figures. I had 14 miles of 34 mpg on the trip meter when I began a 20-mile highway trip that brought it to 41 mpg going mostly 80 miles per hour (some traffic made it 70-80). But that had a bunch of downhills. On the way back, with a reset trip meter, I got 40 mpg from 0mph to 0mph, but not only was it mostly uphill but there was a headwind... weather.com says 12 miles per hour but it felt like more... anyway hopefully sometime I can do a more fair test and compare the two driving strategies.

Clearly S is less fuel-efficient in the short term but over a decent-length highway drive like the one I did, it definitely helped alot.