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Here is a little math work to put that into one money saving scenario:That 9 mpg avg. to 19.5 mpg avg. in my neck of the woods is just under 18 cent/mile decrease in fuel cost ($3.42 unleaded to $3.96 diesel). I drove my truck a little under 10,000 miles last year so that would be almost $1,800 savings per year. This assumes you don't drive it more now because you get better fuel mileage.My 11 mpg avg. to your 19.5 mpg avg. at the same 10,000 miles per year is about 11 cents per mile so about $1,100 per year savings.How much do you think you have in the conversion? Are you running an overdrive transmission? 4X4 or 4X2?Sorry for all of the questions, I am just not familiar with your truck or any conversion posts you may have. Suppose I probably should have done a search before I started this response.
Your fuel economy is not bad at all for all the city driving. These are not exactly light weight vehicles so they take a lot of power to get up to speed. I bet you can get over 30 mpg on the freeway with your combo and driving easy. Best fuel economy is around 55-60 with these square nose trucks. I drive almost all freeway so I can't comment on what kind of fuel economy my 6BT Cummins Suburban would get under the same circumstances as you. However, I'm getting about 25 mpg on the freeway driving easy. I also just swapped out the wide tires for some 265/70/17's so hopefully, I'll pick up a mile or two.I think I remember you saying you advanced the timing. That's helps some.Since you do a lot of city driving, you might think about installing a lower stall converter. Did you install a diesel converter? Those are OK, but there are even lower stall converters out there. The lower the stall, the less waste from slippage. The Cummins has so much torque that slippage from a converter is not necessary for it to build power.
Also, add a little bit of transmission oil or 2 cycle motor oil to your diesel. I use 1 oz of the oil for every 2 gallons of diesel. I heard that it helps to restore some of the fuel economy loss from the Ultra Low Sulfer reformulated diesel.These trucks are also very sensitive to highway speeds. With the high price of fuel, I keep my speed to no more than 60 mph for best fuel economy. Back when diesel was cheap, I used to drive 75-80 mph with my 6.2L Suburban and it would only get 20-22 mpg. However, since then, I have broken 30 mpg driving a steady 55 mph.