Hate to trade my truck with C15 but got a big Mpg education this week with a Volvo

Before I bought my first truck I felt I did a lot of research. I wanted a Cummins or harder to find Detroit but came across a truck that had such great care and outstanding shape but had a Cat C15 2006 Acert with 3.36 and 13 speed. I understood it would be harder on fuel but would make up for it in longevity. Thought it was a pansy so Cat increased to 550 hp. since had 1850 tranny. Improvement in power but not much. Bought a Bullydog that really made her go but my intent is for fuel economy not blowing doors off. BD did nothing for mpg. Even added air dam on cab and nothing.
I get 5.5 or less on IFTA's and have mostly thought people were full of BS on getting over 6 mpg pulling 65,000 to 80,000 flatbed and vans like we do. Well after a guy ran into us head on and my truck was in the body shop I was given a Volvo to use with a 2006 D12 370 hp in it, 10 speed, 3.70 and I averaged for the week 6.4 mpg that's a whole 1 mpg difference and some serious change on 120,000 miles per year!!! I felt sorry for the Volvo as big gaps in gearing and could never own a truck that was forced to lug like that. Really no gear for 48 - 54 mph. Over revved or severely lugged. Nice riding truck but engine braking is not existent and the dash is cheap and cheesy. However it would put a lot more money in my pocket.
I am just sick. My truck is drooled over by many and shops can't believe how good a shape it is in. It gets continued meticulous care by me. I have replaced water pump. Also the input shaft thought the clutch and transmission was a piñata and beat them this spring so now I have a new clutch and rebuilt reman transmission. Truck uses 2 quarts of oil at 11,000 miles and I change it every 12,000 miles. Not a leak on it anywhere. My truck has 480,000 miles on it now. New virgin rubber everywhere. I feel I would be giving the next guy a gold egg and perhaps buying something someone else could not wait to get rid of.


1. Should I just keep it and try to get my money out ?


2. Convert it to a single turbo with kits now being advertised and offered for big mpg gains ?


3. Trade it for something else ??


I am thinking I should pull the twin turbos off but have not heard of anyone who has or has done it for an extended period of time for results etc.

 

International 9400i

 

International 9400i not the best but not the worst

 

Never break 65 mph. Try to never let it lug below 1250. At 65 it is at about 1300 rpm perhaps that is too low ?