The following is from Gregory's service and repair manual no.245
3.9 Litre Engine:
Manual transmission 11.5 (city) 7.5 (highway)
Automatic transmission 13 (city) 9 (highway)
The fuel consumption information is based on tests made according to Australian Standard 2877. The actual fuel consumption obtained will depend on many factors including prevailing conditions, driving habits and the vehicles equipment and condition.
hope this helps, cheers.