Jyden, thanks, but a Scanguage is one more thing to have to buy and wire in. I will resist the temptation for a while longer. I can still see momentary or real time mpg up to 199mpg on the dash, and monitoring my fuel bills and odometer readings is the most accurate way to see overall average mpg. It's just occasional trips that have a displayed average over 99.9mpg on the dash display now. The overall average is reading in the low 90's, although that corresponds to about 80mpg as measured via fuel bills and odometer readings. Changing the display to read km/litre or litres/100km would be free (or cheaper - I'd have to pay $99 for the full version of the VAGCOM software, but I want that anyway as it will enable me to modify other behaviours of the car too, like idle speed and so on.)
As for the fuel log, I have one at fuelly.com. I'll see if i can link to it...
And Clifornia98Civic, that's a great tip about the metal jacket for the Odyssey battery, as my engine bay temperatures do get pretty high now that I have an almost 100% grille block. (Well, I've measured 50 degrees C on occasion, but not much higher than that.) In their sales blurb Odyssey claim an 80 degrees C maximum temperature rating for their batteries, but it's only when you read in more detail you see the maximum is 40 degrees without the metal jacket. I'll definitely look into that. I was under the impression that high temperatures are a problem with AGM batteries mainly when being charged, and since I mainly charge the battery with the engine off (with a temperature-compensating battery charger) I was not going to bother with the metal jacket. Also, the VW Golf battery is fully enclosed in a plastic box anyway. But your experience is leaning me towards taking the issue more seriously. Maybe my existing AGM battery is suffering its premature loss of capacity because of engine bay heat? How much additional space does the FMJ take up? Will it still fit in the battery tray/box in the same way? If not, then perhaps I am better off trying to improve the heat-shielding capability of the existing VW battery box (with heavy metal foil on the outside for example? ...or by applying that Dynamat stuff?)
Another possibility I am seriously considering now is to use TWO parallel 100Ah deep cycle batteries in the spare wheel well, and use a supercapacitor array in the engine bay in place of a battery for engine starting and for other peak current demands. No overheating issues in the spare wheel well. I'd also get a second 12A DC/DC converter and wire it in parallel so I'd have at least 25A at 14v available. (Actual output of this unit is 13.5A in fact, and they are designed to be used in parallel like that.)
Maxwell Boostcaps have a maximum environmental temperature rating of 65 degrees C and should outlive a vehicle by some margin in engine bay conditions. In the first instance I could use a standard, (already owned!) small-ish non-AGM (therefore more heat tolerant) battery under the bonnet and rely on the two large batteries in the rear for cycling, switching the alternator back on when the starter battery/supercapacitor array voltage starts to drop, ...which would only happen very rarely on longer trips as the voltage would remain up at 13-14v for many hours. For daytime driving with no wiper/blower/headlights I'd get 12-15 hours' use at 8A, and even on a rainy night I'd get 3-4 hours' use at 24A.) I'm going to install the first 100Ah Yuasa deep cycle battery in the spare wheel well today and I am going to make space for a second one in there as well, so I can just slot another in there later.