Sum:
Don't know how I missed your response a week ago, but:
We tow out to Bonneville twice in a year, so there's about 8,000 miles, and tow to Maxton once or maybe twice a year, and that adds another 2,500 miles/ trip. Add side trips to Newfoundland, Pennsylvania (Mike, our builder) and add another 1,000 miles -- okay, 14,000 miles year towing. Yeah, we drive a whole bunch more miles to Bville and Maxton -- but without a trailer tagging long behind us.
So now we're up to close to two years to pay back the cost while towing, and therefore it's a valid question to see what difference, if any, I might expect in non-towing situations. Maybe it'd happen sooner than two years, maybe not.
As for Diesel cost: Today it dropped again here in Marquette. I haven't been paying over three bucks/gallon, and now it's a nickel cheaper.
I'm interested in the water/methanol system -- but want to take a hard look at the numbers. I remember doing what I thought was diligent research on fuel economy before I bought the diesel pickup -- and I discovered that I had been given bogus information by at least a dozen folks, mostly owners, as to what mileage I might get. They ALL gave me wildly high numbers -- like maybe the fuel mileage they got on a good day, down hill, with a tailwind, while running at economy cruise on one long freeway. I wanted/still want real life numbers. I have a record of all the fuel I've put in the truck for the past 96,000 miles, by the individual fill-up, and with location of the fuel stop. I can give a real answer to the question about what fuel economy I get -- I want similar answers about what I can expect.
So -- while the system might pay for itself in heavy-work situations, what can I expect all the time? That's still my question.