[Konstantinos]

Yes, that way the loop will continue until c reaches MAX_VEHICLES-1.

Though as I told you before declaring a string with size of 256 2k times and also sending the message 2k times is not that good.

pawn Код:

new stringgas[60];
for ( new c = 0 ; c < MAX_VEHICLES ; c ++)  
{  
    carfuel[c] += quantity; 
}
format(stringgas, sizeof(stringgas), "AdmCmd: You supplied %d liter on all cars of San Andreas", quantity);  
SendClientMessage(playerid, blue, stringgas);

Also vehicle IDs start from 1 so you may start the loop from 1 as well (unless you use index 0).