With a speed restriction like that, I’d have EV mode for town use only. I know when I had the Ampera with its generator, we used the petrol to do the faster driving and switched to pure EV once we’d get to where speeds were never that high. It maximised the combined range, which is now about what I can get out of the Hi5 now.
That approach wouldn’t work for me as we do very little driving in towns and all journeys, even local ones, involve motorways, so if we couldn’t drive our PHEV on the motorway in EV mode at a decent speed it would be a complete waste of time for us!