Calculation of truck consumption

vitoaiaco  first I must say I'm not a fan of you sheet set up. 

a) It appears you insert columns to enter the latest data which can/will throw problems into calculation.

b) you are using columns instead of rows (you have many more rows than columns)

c) you are grouping data/columns instead of having individual entries.

that all said the following formula will work:

=LET(miles, FILTER(D6:AAA6,(D$3:AAA$3="KM")*(D6:AAA6<>"")), 
     gas, FILTER(D6:AAA6,(D$3:AAA$3="Diesel Ltrs")*(D6:AAA6<>"")), 
     (INDEX(miles,1)-INDEX(miles,2))/INDEX(gas,2))

or you could use INDEX or CHOOSECOLS instead of FILTER

That said, you may want or need to add a 'dummy' column D so the formula wont get shifted when you insert the columns.

Lastly I question your calculation because I always do current milage - last milage / how much I need to fill the tank NOW as how much I used which assumes I top the tank off.  Your calculation using the prior gas amount works if you run the engine dry each time (which I sencerely doubt) or I'm just misunderstanding the data.