Tripometer correct?

Just trying to figure out if my tripometer right or is the trucks gas mileage calculator right.
A few numbers...truck says I get 15.6L/100km (15mpg) , my tripometer says 381km(237miles), when I put 75Ls(19.8 gals) in...so my calculation would be 19.7L/100km (12mpg)
I like the trucks figure better but thinking I'm closer to my calculation number?
My tires are only 1.6% taller, so even if that get factors in, the number doesn't change much.
Do you have to calculate tire circumference for mileage? Or is the tripometer correct?

My trucks only leveled, approx 2inches, have a cap and carry around 300lbs...265 70 18 wildpeaks

 

You must calculate for the change in tire, which will change the distance number.