OK, here're the steps:
1. goto each car folder and see if there's data.acd file, if there's this file continue to step 2, otherwise skip to step 4:
2. use Assetto Corsa Car Tuner, in the Select Car combobox, select the car you want to modify
3. enter "1" (or whatever) in New Tune Name: field and click create new tune (data.acd is now unpacked)
4. goto "assettocorsa\content\cars" and find the folder with the name <carname>_1 for example ks_ferrari_312_67_1 which is the unpacked folder for ks_ferrari_312_67
5. go into that folder (with _1 appended) or the car folder (if there's no data.acd and you skipped steps 2-4) go into data folder there should be many ini files
6. create a file named fuel_cons.ini and enter the following text (an example is in the attachment)
[FUEL_EVAL]
KM_PER_LITER=1.6
7. save
8. try your car in AC to see whether AI takes the proper amount of fuel. if they take too much fuel increase the value for KM_PER_LITER, if they run out of fuel decrease the value. NOTE: if you unpacked data.acd make sure you select the tuned car, instead of the original one, it's created as car tune for AC. (I would say you can move the original car folder somewhere else to avoid confusion, )
9. do this for all cars
Assetto Corsa Car Tuner can be downloaded here:
https://www.racedepartment.com/downloads/assetto-corsa-car-tuner.13946/