It wasn't until recently that I broke the 80mpg tank barrier. I've been on an upward trend which I think has more to do with becoming familiar with the car than with any mods I'm doing to it. I think largely it comes down to two things: 1) identifying when the car is at its most efficient, and maximizing time spent there, and 2) identifying when the car is least efficient and minimizing time there.
Here's a BSFC chart for this car, with the area I spend the most time in marked in red.
One point in my favor is that Vermont is naturally conducive to high fuel economy. We have mountains which tend to hurt FE, but the vast majority of my driving is done at 50mph, which is ~1825rpm, and at those speeds I'm at about ~80-100mpg in lean burn, which is about 70-80% load and a perfect bulls-eye in terms of efficiency.
I've noticed that when I lean on assist to climb a hill, I pay for it later. Using a little assist for a short time is good, especially if it can keep me from downshifting out of that Goldilocks zone because I get that energy back with regenerative braking, but using enough that over time the battery's SoC drops and the car decides to background charge while driving at speed
seems to hurt FE. My best guess is that it's because when you convert from mechanical energy to electrical, you lose X%, then when you put it in the battery you lose X%, and then when you take it back out of the battery you lose X%, and when you convert it from electrical to mechanical you lose X% again. So, mostly I try to keep the battery meter between 70-90%.
When I need to climb a hill, if my battery is close to full I'll use assist and bleed off a bar or two, but more often I'll hit my calpod switch, climb a bit in 5th bleeding a little speed then drop into 4th, and if it's steep bleed more speed and drop into 3rd, to avoid running my battery down and help stay in that Goldilocks zone of high efficiency. I'll generally lean on assist rather than rev the engine up past ~3000 in 3rd to climb.
When it comes to slowing/stopping, I generally prefer to regen rather than coast with the engine off. I figure keeping the battery up is equivalent to an alternator delete (keeps load off the engine). I'll throw it in neutral and hit my kill switch if the grade is just right that I maintain speed, but otherwise I leave it in 5th and let the battery charge. When coming to a stop, I'll regen down 30 in 5th, throw it into 3rd and regen down to ~19, at which point pressing the clutch initiates auto-stop and I can coast the rest of the way to the light or stop sign.
I'm still on the fence about whether or not lean burn at higher RPM is better or not. I really don't know if BSFC is lower in lean burn, or if it just moves where you are on the chart, so I generally try to stay between 1750-2250rpm and go lean where I can.
Other than that, it's a lot of common-sense hypermiling: Drive as though I don't have brakes, think about what's in front of me, and keep my speed low(ish).
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So, this most recent tank was right around 70mpg. Almost all of it was done driving in Boston traffic, either stop-and-go in the city or on ~20 mile stretches of highway where people are either doing ~70, or slamming on their brakes. Temperatures have been between 30 and 50F depending on the time of day. I've found the biggest challenge here is keeping my speed low in a way that isn't unsafe or uncourteous to other drivers. There's usually a lane I can get away with it in, but generally I've been driving at 55-60 here and occasionally as high a 65 in the slow lanes, because I consider safety to be more important than another 5-10mpg. It's "fun" to get into a stop and go situation though and move at the average speed of traffic.
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