Buggered if I know how many milk bottles of fuel my Datsuns consume per hour of freeway driving, but my B10 sedan with big bore GX engine was fueled up in Albury one night on a trip to Bendigo & we averaged over 72mph [116kph] for the 250 odd miles [402k] & it sipped the fuel at a rate of better than 42 mpg [68k/4.54609 litres]

On another occasion I 'drove for economy' from my place to Holbrook for the Ultralight Fly-in [1995] & it returned better than 51mpg [82k/4.54609 litres]

Remember that this is for Imperial gallons, not US gallons, which are 20% less & my engine was a near stock 1200 GX. This engine was running a 76.5mm bore as the only significant departure from standard. 5th gear was 1 to 1 & I used a 1200 diff of 3.9 to 1 ratio.
Rear tyre size was either 195x65x13 [583mm diameter] or 205x60x13 [575mm diameter] against the stock 600x12 size of 574mm