I think the only reason other than the cat an eco could be getting worse mileage with the gov mod is the trans they used. I think it had 4.20:1 ring and pinion gear in the trans they came stock with. So if the gov isn't cutting fuel and you are reving like crazy it probably is using more.
I know everyone here loves to hate on the AVX, but I just don't think turning 3-400 RPM lower is worth the time and money hunting down another transmission to swap. It's like the 5th gear swaps for the TDI's, they cost like 300 bucks, 300 dollars will buy a lot of diesel and you'd have to drive a ton to recoup getting maybe .5-1 mpg (?) further. And that's just it, who really knows if it makes a measurable difference running 2600 RPM vs 2900 RPM is an ECOdiesel at 60 mph. There are a lot of variables to consider that only the Germans who engineered this stuff have answers to. I would love to pick one of the engineers brains and just ask him, "Why didn't you just put a tranny in there that turns 2000 RPM at 70 mph so I could get 70 mpg with it?", since that seems to be the way of thinking lower RPM= more MPG. He would no doubt laugh in my face. I imagine he'd say that there is a compromise of RPM, power, and fuel usage that breaks down somewhere in all the BSFC charts, thermodynamics, yadda yadda and that they built the ECOdiesel the way they did for a reason. I know the AVX is capable of 50 MPG tanks consistently for the type of driving I do, which is good enough for me. I would love to swap the 4A in since I already have it and it is 99.9% ratio comparable to the much touted 3.67/.75 combo trans, but I am just not able to at this time. If I ever do I would love to do some mileage testing to get some answers on the AVX vs 3.67/.75 mileage debate lol.
This last round of driving for work was all over flat and windy west Texas, nearly 2000 miles 95% highway the last few weeks. I did a few tanks driving my "normal" way (like a grandma) 55-60 a whole tank, got around 41-42 mpg. I then tried a few tanks with more spirited and faster going 65-70 the whole tank, mileage was still the same 41-42, very consistent. I topped up into the filler neck till I could see diesel everytime, and ran it to just below the red everytime so my methods for calculating mileage were consistent. The worse tank of the journey was the first one, driving into a 20 mph head / crosswind for about 250 miles with rain. That tank I averaged 38-39, one of the worse tanks ever recorded in the car. Wind definitely plays a factor.
Real world testing has shown going slower nets more mpg especially in stock form, but it just felt like to me with the governor mod my static fueling was just locked in to a set amount no matter how I drove. It had the power I guess to get the same mileage at higher speeds, but when I went slower it just didn't matter cause of the gov mod and max fuel screw adjustments just kept fueling too high... Another thing when I put my stock spring back in, my RPM's hanged when I rev'd it so had to back out the max fuel screw to get it acting normal. I had the max fuel screw in quite a bit perhaps was my issue instead... I remember having to turn it in quite a bit to get it running right after doing the gov mod...perhaps my shim was too extreme? I could barely get that little E clip back on with my solid socket shim on the governor.
Something else I notice too is if I run a tank without additive (grey Power Service), my coolant gauge will read just slightly lower than the middle and will barely rise to middle on a long hill pull or something. It would heat up very quickly if idled for a bit too. But if I run a tank with additive, it will be dead-on straight up and down in the middle while driving where it should be, and won't seem to get as hot while idling for extended periods. Something to do with cetane and combustion efficiency I think. I always fill up with the absolute cheapest diesel I can find, which was usually Wal-mart (Murphy's) with the extra 3¢ off if you use their gift card to buy fuel. So probably not the best quality higher cetane fuel, but with an additive it seems to run in the more sweet spot of the efficiency range.
You can swap the longer delivery valves into the other pump, but you will need to adjust the max fuel screw by a fair bit.
Thanks for this info.