Your gas mileage was 9.78 miles per gallon.
The ideal mpg for a 1991 LS400 is 18 City and 23 Highway:
http://www.fueleconomy.gov/!Removed!/compx2005f....nField=Findacar
There are many factors concerning a vehicle's fuel economy many, if not all, of which have been discussed in previous threads concerning poor gas mileage such as: tire pressure, "heavy foot", etc...