I think you might be posting in the wrong forum...
The TDI engine is only available in north america in '96 and later cars I believe. The 89 would not be TDI, so a post in the IDI section would
probably get you some good answers..
Anyway, I get about 49-53 MPG on the hiway depending on
conditions with my 99.5 Jetta TDI. Thats if I set the cruse for 78 MPH or less.
Heavy head winds or heavy foot will decrease that however..
Drive ON!