Imperial gallons are meaningless in the US. We don't use them. 4.2 L per 100 km is ~56 miles per US gallon 4.0 L per 100 km is ~58.8 miles per US gallon I'm surprised people are still responding to this thread. It seems the OP is no longer an active member and really, the numbers will vary a lot depending on speed.
I just got back to L.A. from Vegas as well. Set cruise control anywhere from 70 to 74 mph the whole way there and back, but mostly at 71. The trip meter is saying the mpg was around 46 in either direction. Given that trip meter is generally a couple mpg generous, I might not have done any better than you did. Maybe next time I will try 75+. But the faster I set the cruise control, the more likely I have to brake and then re-set it when the lanes get choked...
