Thanks for the feed back to my post on Friday about car efficiency, where I commented on progress towards getting over 40mpg with the car I have now. And the happy progress past the 6L/100km mark.
Skippy - checked air filter within week of purchase.
Tyop - check tyres weekly. Have to add about 2 kPa per week to port side rear tyre. Annoying little leak.
others - I'm still happy to hear more ideas on how to reasonably get efficient urban transportation from a conventional automobile. And in support of that, here is what I have done so far:

The first week I had this vehicle (new to car and had previous owners adjustments/ lack thereof) it ran at 26 mpg... after I checked and corrected the tyre pressure, the handling greatly improved and I got 33 mpg. After a silly amount of research to determine safety effects of moderate overinflation, I went from 28 psi (owners manual: 27) to 31 psi forward, 30 psi rear. Got 36 mpg. I also reduced the kit in the boot to minimise weight during the commute.

The remainder of the current improvement to 39.5 is due to short-shifting and turning off the engine while waiting for trains: I try to keep the car under 2k rpm off-highway, under 3k highway, and do as much acceleration under 2.5k as possible, preferring 2k if possible. But the occasional nasty uphill with a short ramp merge with highway speeds at the end I'll still take at 5k to 5.5k rpm, 'cause I value my neck. (redline is about 7 - it's weird how the performance changes when I just go to a different shifting pattern).

This week (and next, since it takes that long to use the fuel) I'm running fuel-injector cleaner through. I expect less efficiency while it runs through, but more afterwards.

In May, I'm hoping to correct the alignment, followed by newer tyres with better rigidity (and thus a more aerodynamic cross-section) will help a little, particularly the alignment. Once I have them, I'll tune the pressure to the maximum that still has even tread contact across the face (determined by the wear on a thin line of paint across the tyre), assuming said maximum is within the rating of the tyre. I'm also thinking of taking the time to clean the contacts in the electrical system, so the battery can stay slightly more charged (and thus present a higher back voltage to the alternator, reducing it's demands on the engine)

It appears to have straight emissions tubing and piping - is there something significant that can happen there other than crimping that increases outlet resistance from the engine?`

Two further unanswered questions I am pursuing: is it more efficient to coast downhill in neutral (engine idling, so just a little fuel) or in gear (higher revs, but might not require any fuel)? And at what speed is the peak engine efficiency? It seems like about 1.7 to 2.2 rpm for the latter, but have not been able to confirm yet. I expect it to be around half of the peak power output.

I've also considered keeping a "battery maintenance" solar panel (like the $12 ones from Harbor Freight) plugged in to again keep the battery topped up and lighten the alternator load. But I have decided that's more of a be-able-to-start-in-the-woods safety thing than efficiency thing, since 1.5 Watts works out to something like the energy of one three-thousandths of a gallon of petroleum distillate per hour. Not a particularly useful gain there. I think. Unless recharging from the alternator is really inefficient for some reason.

My amusement value in all this: the only concrete number I've encountered for a hybrid in use as a commuter car in the US was 41.5 mpg. I'm thinking I may be able to beat that in an older conventional car, just by careful maintenance and use. Even without spending the extra for synthetic lubricating oils...

Added bonus: the accident rate for people driving like this is a LOT lower (better reaction time-to-velocity ratio, better water displacement in rain, less v^2 being sent to heat the brakes, and fewer insufficient lubrication/cooling engine tragedies), and curiously, so is the fatality rate when an accident does occur (lower impact energy and less chance of tyres separating from the wheels in a skid).