Here is my fuel log, my nemesis are wet and snowy roads.
1997 Citroen ZX Monaco Gas Mileage (ZX) - EcoModder.com
Your car has no OBDII afaik, they probably are not plugging anything to plug that is under steering wheel at inspection either? They test OBDII cars there so that is one way to tell.
Boost gauge is from my experience quite helpful, it is also quite easy to install as there is tube coming from intercooler to pump, you can easily put T-piece there.
Every downhill my temps are falling at this weather, I might put some blocking to upper grille too.
Wiki (link is at top of this page, black small words menu that is easy to miss) has explanation of testing method how to get cd and rolling resistance tested, level road is needed for that.
There is also another method posted at here by tarksu:
Ilmanvastuksen että vierintävaskuksen mittailua
I think that your car was also sold as Citroen, some small changes perhaps?
With testing, weather variable, especially wind can have lot of effect, so comparision between the runs is always bit of challenge with road testing. More test should give more accurate results however, so I recommend to do many runs and work out average from those.
With Peugeot they might have used newer model engine in those? I remember 306 did move to electronically controlled engine at some point while ZX never got those, only after Xsara replaced ZX they got newer model engine, HDi.
I have Volvo 240 too, diesel version, has been off the road few years. With serviced brakes and new Nokia summer tires it did coast a lot worse than my ZX, I think it was because of tires, but aerodynamics probably did play part in equation too.
Oh yes temps, well my temp meter is not made for easy reading, but normal is just needle thickness away from 90C. Motonet does sell only 82C thermostat, I bought mine from Citroen stealership, was not expensive and I think this is 86-88C more engine temp should be better for economy I have read.
Before that it was just bit above 75C lower when coasting, now it is that 86-88C, but when I coast down a hill it can drop close to 75C.
Genuine Citroen thermostat is good in there that it stays closed until temperature is reached and opens then fully, I can observe this from temperature meter, it goes close to 90 first, then drops all of sudden from there, it does make car heat up lot faster, old one was not such type.
My efan is broken, so summer I have to keep eye of temperatures, but as I don't drive at city it really is not bothering me, fan is not really needed, especially when I turn engine off at traffic lights.
Air filter in mine is foam that needs to be oiled, there is paper versions sold to this model too, but XUD9TE should have that foam filter, it is bit noisy for my liking, but there is less restriction and that might do something to economy, not quite sure about it.