Toyota Repair: diagnostic, better mileage, rush hours

Question
Hi, I am concerned about the big difference in fuel consumption of my car. It is 1993 Toyota Previa, AWD, 173,000 km.
It will make only 300 kms on a full tank (65 litres) in the city (almost all driving done outside of rush hours with only few stops), but it will make 550 kms on the freeway. It is a huge difference, does it suggests any specific problem? My driving style is very no aggressive out or in the city. Is there any diagnostic test a Toyota mechanic could run and then read some codes stored in the computer memory which could reveal the problem? Thank you for your help. MIlan

Answer
Its not OBDII so you cant just hook up a scanner.
whats thwe actual difference?
I have a MPH speedo, and buy fuel in gallons.

I own a 95 and get much better mileage on highwayn trips also. Its a heavy vehicle fo a 4 cyl.
On the highway youve overcome the weight already and spend alot of time in OD.


Mike