At 49.1mpg you'd have been in the 800 mile club if your tank had been full.
charcoal canister, on a diesel??? no worries there... on the Audi, yes. MkIV TDI, no.
Anyone feel free to correct me if I'm wrong on that but I'm pretty darn sure there are no diesel fuel vapors to collect with a charcoal canister.
on a gas car "over-filling" -can- cause such problems but again, those types of worries like electric in-tank fuel pumps and emissions charcoal canisters and such don't matter on our ALH powered MkIV TDI's.
I've run my previous TDI MkIV and this one down to just under 17 gallons many many times and haven't had any "air in the line" problems from it yet. I've never run mine dry, but I've had my share of "loss of prime" issues from the car sitting parked with old cracked injector return lines and bad pump seals and such on each of my two TDI's when I first got them so I've had to re-prime my lines more than once until I found and fixed the problems, and I know my current car's previous owner ran it out of fuel more than once, and it wasn't a big expensive ordeal although she had to pay someone to re-prime her lines so the car would start again.
Yes, your automatic has a bigger injection pump and one less gear than the manual cars but it sounds like you're still managing to get stellar economy out of it anyway.
I didn't notice any power increase with my intake manifold cleaning, and trust me I'd notice that (agressive driver), but it did seem to improve my economy a little bit, although mine was so clogged a #2 pencil wouldn't fit into the intake opening at all (hadn't ever been cleaned in 149,950 miles since new) and the car still ran and drove totally normal.
Ironically, as for the 6-speed, lately I'm rivaling one of my 6-speed buddies and beating the other with a stock 5-speed car at the moment, we have no idea how it's happening and I'm not actually trying...
Add that to your "diminishing returns" list, sadly...
The 6-Speeds make about 4k rpm's difference in top gear at 70mph so I'm theorizing that since I somehow got 48mpg without trying on my last tank and I'm on track for it to happen again on this tank driving at 70mph, if I slow down about 10mph on my commute I may cross into the low 50's for mpg's and get close to the 900 mile club. I was going to try it on this tank since my last one was my first 800 mile tank, but I got a work emergency call the morning after I refuelled and had to get there in a hurry so that call blew my chances of a 900 mile effort on this tank and that emergency situation lasted 2.5 days.
Both of my 6-speed swapped buddies have 2 different transmission models and both see about the same results from them. They're now saying they really like it but it'll never pay for itself in fuel savings and it's expensive to do the swap so it's not actually worth it if you're doing it for the savings...
I was seriously considering it myself because I don't like how close together the first 3 gears are on the manual 5-speed and I thought the 6-speeds would give me taller ratios in the lower gears so I could stay in each of them them longer (higher speed per gear) but apparently the 6-speed is basically geared like the stock 5-speed with an extra overdrive position which wasn't really what I was looking for and my stock clutch is still working properly so I decided it wasn't worth the extra unnecessary cost and work to possibly gain a few more mpg's and maybe 50-75 more miles per tank.
Again, I'm sure there are those who have done that swap that will justify/defend it as the greatest mod ever but at 65-70mph with minor basic modifications or a stock car it's typically not like some magic 200+ mile per tank increase by having that extra gear...
I see owners in the 800 mile club with a stock ALH MkIV that used a gallon less than my car with a few minor mods did to get there which is 2mpg better for a 35 mile extra available driving range beyond what mine did. Not a huge difference.
Heck 5mpg extra x17 gallons only gets 85 more miles... 10mpg x17 gallons is 170 miles and I doubt a 6-speed swap itself will ever provide a 10mpg increase.
Return on investment vs diminishing returns.
Pardon the minor rant...
Of course you're up north and I'm waaaay down south but when I got this car everyone told me the dead (stuck wide open) thermostat was killing my economy but it took 6 months to get the car's title from its' previous/original owner and the car had some safety/mechanical issues when I got it so I ignored the bad thermostat for like 10k miles since it wasn't throwing a CEL even though my Scan Gauge said my temps were only getting up to 135-160f at the warmest point most of the time in city idling and on the interstate, and I was all excited about some big economy increase once I got the title and began doing some of the maintenance stuff because "everyone says a bad thermostat kills your economy", but it didn't make a noticeable difference in my driving distance per tank with the same amount of fuel as before at each fill-up so don't go getting your hopes up about a warmed up engine increasing your economy.
I'm sure many people will dispute that but that was my personal experience and I doubt if it gave me 1mpg improvement if that much, granted again I'm down south and I drive at the posted speed limit and mostly on the interstate (70mph around here) so for those old farts driving 45-55mph everywhere it might've been a huge improvement but for those of us doing normal driving it was a total wash...
I agree that spending so much time and money to try for higher mpg numbers is a diminishing returns game, albeit a fun one at times I'm sure...
and the 700 mile, 800 mile, and 900 mile clubs are a game too... which is a fun one to play for 800 and beyond but not on a regular basis as the necessary driving to attempt the 900 mile club in my car would drive me nuts to have to do on a regular basis...
Heck, just going an entire tank driving 60mph is challenging for me as traffic is flying past at posted speeds or higher and I've almost been rear-ended on the interstate several times by inattentive speeders while I was going slow to try for higher mpg's.
If you live in an area where the posted limits are in a more optimum economy range for these cars then it's no big deal but watching traffic pile up behind you to save a few bucks is more stressful to me than it's worth in what minor savings it gives per fill-up...
One thing that did change on my car on the tank where I ended up in the 800 mile club was that I added a skid plate with around 100 miles on the tank so I'll be curious to try a few more experiments over the next few tanks and see if it contributed to my recent significant economy improvement or not. Obviously being able to not use a/c as the weather cooled off is helping but 5mpg above the previous tank with a/c always on seems drastic. Literally went from 43mpg with a/c on to 48 with it only used for about 1/3 of the tank as-needed and a skid plate added while driving the same agressive way and same posted speeds for the same work commuting on both tanks. If the skid plate did contribute to the major increase then it's totally worth it as it also protects your oil pan in areas with crappy roads and potholes and road debris like we have around here. We'll see how my next few tanks do as my last tank, the 800 mile tank, is my first with the skid plate installed on the car.
NOTE: both of my 6-speed buddies have skid plates and I didn't and they kept insisting my driving habits were keeping my mpg's so low. Now I swapped my wheels back to AVUS' and did 40psi (they run 45psi with TPMS') and added a skid plate and magically I'm right up there in a very similar economy range as them for mpg's in my 5-speed while driving the same as before, at (or slightly over) posted speeds and moderately agressive... Nowdays one of them beats me for mpg's and one of them doesn't usually.
Now the game is on... 900 mile club is my goal as soon as my work schedule allows me to drive slow enough to try for it...
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