I'm getting 13.2 mpg on my 03 6.0 1500 hd. Is

You'll need to know how big the fuel tank is on your truck. They used different sized tanks depending on the box or body fitted. I think the standard body box was a 26 gallon tank and the long body box was a 34 gallon. This will help you work out your actual miles per gallon.
That model's factory figures have it at between 12 to 13 mpg average. How and where you drive your truck can also affect you fuel consumption quite a lot, City driving, Hard accelerating, Lots of steep hills where you drive and Driving Off Road all can ruin you MPG figures. Using the your numbers that does seem a little lower than it should be. When was it last given a full service? Have you changed fuel supplier, (different gas station). All of these affect MPG Do you have any warning lights on the Dashboard? Use the comment button below with more information and I'll try to help you more.

Using your numbers here's the math.
26 gallon tank - half tank = 13 gallons. 34 gallon tank - half tank = 17 gallons. You got 104 miles.
Sum used is 'Miles achieved divided by the number of gallons used'.
(26 tank) 104 divided by 13 = 8 Mpg.
(34 tank) 104 divided by 17 = 6 Mpg.

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No way to know this, to many variables. But based on other vans it is normal.

It was an already poor gas mileage, now it has goon even worse. Check the filters and the diesel pump, it must be cleaned and adjusted on the testing bench.

Having owned and thoroughly enjoyed an 01 Durango SLT plus for 6 years, congrats on having the "D".

How many miles on your D? Aging O2 sensors can cause mileage to drop. Which engine do you have? The 4.7 or the 5.9?

You will pretty much find that 11-13 mpg in town is the norm, and 17 is normal for hwy cruising. Towing though in either case will have quite an effect.

Combined fuel ecomony is based on 50/50 driving between city and highway. But I agree that 13 mpg is quite a bit down from that!

Do you have a "check engine" light staying on while you drive?
I am guessing you don't or you would have mentioned it.

Bad fuel economy with no warning indicator can be a sign of problems with low fuel pressure or a non-functioning coolant temp sensor.

The coolant sensor can be reading in the cold range and keeping the car in a "cold/starting" condition that won't allow for the car to adjust to being warmed up.

I would have the car checked for any error codes by a local shop or parts store that offer cheap or free scans. At least to rule out the coolant sensor or fuel pump. The fuel pump will eventually go completely out if it is going bad so better to know now than later! Fuel pumps are expensive to replace and you will want time to save up for the job!

Good luck and be careful.

The better the engine breaths the better the mileage. Was the new computer a stock computer or performance?

go a step ahead, it may not be the issue, but it will help.. replace the spark plugs and coils.

As far as the fuel consumption goes a can of fuel injector cleaner in the gas tank may help unless the injectors require more.Your oxygen sensor may not be working right and if it isnt it tells the car to run richer than required.When was the last time you had the ignition system checked out,Plugs cap HT leads rotor.Not too sure about your fuel system but if you do attempt to remove any part of it you must release the stored pressure carefully as it will spray out quite hard.Watch out for your eyes.