if you put 18 gallons into a 24 gallon tank, you probably still had about 6 gallons (1/4 of the 24) somewhere in the tank... since your electric fuel pump is running, you might have issues with the siphoning pump... pull the cluster reading of the fuel up and monitor your fuel usage as you approach the 1/4 again... if the remaining 6 gallons stay in the left half of the tank, you get the 1/4 tank reading but the car starves for fuel, you have issues with the siphon pump...