Varies greatly but 45-55mph, I never go faster than 60mph unless downhill, regenerating or demonstrating the car.
Say 45-50mph on a long distance trip on A roads, 56mph on motorways etc the time factor does not bother me.
I don't drive it like a normal person !!!
I use the throttle like a cruise control based on mpg rather than speed.
Prior to my phev upgrade I would set the throttle at 100mpg as shown on instant fuel consumption gauge. I then drive at whatever speed the car can manage at 100mpg, slowing down when climbing and going faster when going downhill.
In good weather 100mpg could equate to a road speed of 60mph+ on the flat, but in mid winter it might only do 40mph at 100mpg. Either way I'm not bothered.
When I first did my conversion and in the bitter weather we had, I moved my mpg point to 125mpg for the first few weeks.
Now it's warmer I drive with gauge at 150mpg as much as possible applying the same principle as above, but with a more electric assist in the background to offset the decreased fuel usage.
You still need to burn a minimum quantity of fuel or the engine never gets warm. There is virtually no wasted heat now from the cooling system. In the cold weather with heating on, engine temp got above minimum reqd but never got near thermostat opening point on my commute. So nothing was being wasted. If the stat opens or rad fan operates you are wasting energy. Every little helps.