Total Highway range is my biggest disappointment with my PHEV Clarity. Like the original poster, I find that I get around 225 to 240 miles on a tank of gas, running strictly highway miles at around 75mph in HV mode. On route 80 in PA, if you're going any slower, you're typically not flowing with small vehicle traffic. Anyway, I don't mind getting around 40 MPG in HV mode, but only having 3 hours of drive time between refills seems a bit under-engineered.
On the other hand, for local driving in EV mode (errands, etc.) the clarity is fantastic and I can go months without needing to fill up. So much so that I get concerned about the fuel going stale. I've actually switched to name-brand, premium fuel to mitigate any risk of cheapo fuel damaging the engine if it ages too long. With as little fuel as I use, I don't mind paying the premium price.
Luckily, I don't do a lot of long-range driving so the trade off is acceptable. But I do wish the engineers had found the space to add just a couple more gallons to extend highway range.