how many km per full tank of gas i am not positive but i think i have the 66 litre tank but don't hold me to it i have the 3.9 l engine single cab with just front seats no rear and dual exhaust. lowered if that makes a different i have a fairly heavy foot but not too heavy i get probably around 300 km to the tank and thats ridiculous

 

i am not very good with the conversions off of the top of my head, but it doesnt sounds very good. when was the last time you did a full tune-up? are you running a stock style air filter? what type of exhaust do you have? unfortunately, the 3.9 liter gets pretty sloppy gas milage as it is. the motor is very underpowered for the weight of the vehicle, in turn producing lousy gas milage.

 

Hes getting appx. 10.9 MPG
(Conversion is as follows: 1liter=~0.26gal, 1km=~0.62mile)

As virused said, if you haven't done a tune-up in a while, it would probably be a good idea to get one done.
Of course, the type of driving you do will have a big impact on mileage. If you do mainly city driving, with a semi-lead foot, your gas mileage is gonna suck. (I do 90% city driving, with a some-times lead foot, and average 12MPG; ranges from 11-13MPG)
The quality of fuel can have an effect. If you always by the cut-rate fuel because "its 2 cents a gallon cheaper," that could be a contributing factor. My grandparents have experimented, and spending the extra 2 cents a gallon is worth it.
Since you used "kilometers" and "liters" I will assume your from Canada and the following probably won't apply to you (right now at least) but running the A/C, of course, can/will bring mileage down.

I'm sure you know this, but the only real way to track mileage it to fill your tank up until you can physically SEE THE GAS in the filler neck. Than, next time you fill up, do the same thing. Take the total amount you traveled (for you, kilometes) diveded by the amount of fuel it took (in your case, liters). You can't believe a the computer (if your truck has one), and not all gas pumps kick-off the gas supply at the same level (my tank took like another gallon and a half after the pump kicked itself off).

 

ya thats true, my old ford would pop and i could put up to 3 more gallons in it till it overflowed. My dakota pops and i can put about 1-2 gallons more. Ive got a bad lead foot and i would say 40% city 60% highway, but the traffic here in fortworth is horable so its like the city on the highway... i get about 14-16mpg i useally get about 230-250 till im on E and i still have 4 gal left when my gas light comes on. I Use 93 octane and have a v8 5.9L

 

well i got a 3.9 in my dakota so on a good run for 90 miles... i get about 24 mpg doing a constant speed run... thats doing between 75-80 mph.... but going 25 miles to and back from school each day going on the highway then the side highways and back roads.... i get 8 mpg .. hahaha