I just got back from a trip to Hamilton, Ontario. I the way I stopped and filled my car with 91 octaine and set the trip meter back to zero.
On the way home I stopped again to fill up since the reserve light was on. The first fill was 46.84 liters and the second was 58.11 I changed my fuel tank to the 82 liter version last year so I could extend my range.
The trip between fill ups was 546 km which is 338.5 miles. 58.11 liters is 15.2 gal. US or 12.74 Imp. This gave me 22.27 MPG US and 26.56 Imp. It was almost all highway driving at speeds ranging from 70 - 80 MPH. On my car with the 5 speed, 2.5 liter engine, and the 3.92 axle, I was running at 3,500 - 4,000 RPM most of the time. I've seen as much as 28 MPG at 90 - 100 kmh below 3,000 RPM.
I found that when I bumped my timing up to 37 degrees the mileage went up and so did the performance. When the car was new it was set nearer to 40 degrees by MB so there's more room but 94 octaine isn't as easy to find and I think it would need it to run properly up in that advanced timng range.
What sort of fuel consuption are you getting?