I think most people forget how inefficient older cars were for their size. A 173 LC/LJ was lucky to get 22-23mpg normal driving which is about 8km/L or 12.3L/100. My manual VY SS ute used to do that easily (even better if more highway use) and if I take it easy on my 2 tonne V6 auto Hilux it'll go close to that too. Stick pollution equipment onto a 202 as per HX-HZ with a 1:1 top gear and a 3.55:1 diff which sees it revving at 3200-3500rpm at 110km/h and they drink fuel like there is no tomorrow.

If you want economy in an older Holden you can't go past an EFI V6 conversion, even an EFI 304 with overdrive box will be as efficient or better as an old original 202.