Real world numbers

4.8? Really?
Im sorry I cant help with your question but you would have to think the cat would do better.

 

Well the OP thanks for the real number.

I'll wait for the super truckers to chime in hear with all the 9mpg trucks and all the newbie's that swear their Crapcadia gets 8mpg pulling 79k. You know it must be true because they check it by 'hand.'

 

Pulling a flatbed you should reasonably expect at least 6mpg. Our company has a 379 Pete with the same specs, pulling a tanker in the mountains of West Virginia getting 5.8 on average.

 

real numbers? well,we had guys that couldn't muster 4.5 mpg in our acert cat powered Petes,yet swap drivers and the same truck shot up to 6.5+. of course,telling a truck driver his low economy is self induced always boils down to an argument. even when you show him what his truck was doing with the other driver.

so barring some problem with your detroit-which very well could be the problem-,i would expect your economy to go down with the cat.

i don't have a 9mpg truck but it's a solid 7+ with 45K in the van. it gets that no matter how low the other guys economy is.

 

I had an 05, same specs other than the rears, I had 3:36's in mine, pulling a flat that truck would see 6mpg average unless I was in a big hurry or had a lot of idle time.

 

Geez. My 00 fld 500 detroit with 13 speed and 3.58 on lp 24.5 is at this month 6.5. And lm not happy. 6.5 was yearly average before i let detroit overhaul it. 6.5 to 6.7 is best one time fillup i have seen yet. And 5.2 or 5.3 in cold. Before over haul my best ever was a 7.293 one fill up. Detroit gods were all helping that tankful. That is on paper. I havent calulated yearly yet but probally about 6.1 or so. Oh and stock program with 518hp to ground.

 

I have a 05 century classic 120 frt 12.7 liter Detroit. At 80 mph with 40k plus I get 3.8 to 4.0 mpg going from my to phoenix, my truck never shuts off unless I'm home.

I pull dry van. I don't look at fuel milesge I look at speed limits. I don't care if I can get 9 mpg at 45 mph. My frt pays me to run the speed limits and I'm not counting pennies to stay in business.

 

Ive gotten 6 and change in a 95 FLD. If I took it easy and made that 500 detroit sip fuel as I did a few times I can get 1500 miles and change out of a load of fuel.

Fast forward to a 2001 century, this time same Detroit on auto. Solid 6 and team no chance to sit and burn fuel.

Remember any time you idle for sleep at night it's a few gallons per hour.

With my experience which is not anyone else's experience usually a thirsty truck is either driver issue, too much foot, burnt engine, or likely a fuel leak or rarely theft of fuel.

I used to have a car that I expect and got 30 miles to gallon, for a few months it was short two gallons at 25 mpg I put a locking gas cap on there and suddenly no more shorts which reveals the cause as we slept at night.

 

Counting pennies? Im counting 1/4 pennies. But i dont idle. Under 300 degrees on pyro shut off. But i also dont go to work if it dont pay well. My personal quote. (Just because I drive slow, dont mean it is slow.)