Well I will say one major difference in terms of location--winter. No way can you deny the effect on a full year's driving for having temperatures down below freezing for a few weeks, like I did this Feb. FE dropped down to around 30 mpg for my worst tank, and just a few others in the lower 30s dropped my average for a year down to 35 mpg, despite starting out upper 30s to 40-ish for most of my tanks.

I do think there's an effect of hilly vs flat, but without some serious and exacting study I don't think it's easy to define. And I'm sure it varies a lot by driving style. Up hills is always going to suck more gas than you want, and I'm not sure the downhill coasting compensates for it, so I'd tend to think flat will get better results--but then again my recent mileage, my current tank averaging about 46 mpg nominally (2 commutes to/from work so far) is including at least one fairly major though gradual hill (actually a little hilly stretch that kind of peaks then comes back down), and with a little careful driving it's still possible to keep it in EV mode for a decent chunk and minimize gas. Or I wouldn't be seeing 45+ mpg overall. Both directions.