10 point: 耗油量 about lexus es300 1993

Because you are using "mile" and "gallon" as units, I assume you are using the car in the US, so I am providing you with USA EPA facts.
For 1993 Lexus ES300, EPA estimation for City/Hwy gas mileage is 18/23. The car has a 18.5 Gallon/70 Litres gas tank.
Depends on how you drive the car, if you are always in traffic jams, 105 miles for half tank is not too out-of-sort for a 15 years old car. Toyota/Lexus's tank reserve triggers at around 3 gallons left, so (18.5 - 3) x 50% = 7.75 gallon. 7.75 gallon x 18 mpg = 139.5 miles. Also consider that EPA's number back then is ultra-generous.
If you are still concerned, take your to the garage and ask them to take a look at the cylinder compression. The ES300's 3VZ-FE V6 engine is infamous for gasket leak which cause compression ratio to drop, causing power loss and increase gas consumption.