I worked out mine a couple of weeks ago on a 313 mile round trip, 200 of which was 80-90 on the motorway and the rest booting it on A-roads and driving in traffic around Birmingham.
30 mpg (well 29.7 but that's 30 in my book!), was well pleased with that considering the woes I started my GTV ownership with. My 1.8 MX-5 Mk1 used to do the same trip at 33 mpg, or thereabouts, so not noticing any particular drop in the wallet department.
I guess it has something to do with the two new recon heads? Was thinking the ports would be cleaner etc.