In Toyota Prius and Ford Fusion Hybrids, at what max speed can it go with only Electricity?
For latest models 2017 and up
Prius Gen4 (2017) apparently can go up to 72 mph with electricity only: https://priuschat.com/threads/what-is-the-max-ev-speed-no-ice.161545/ (message #5). However, that doesn't mean the tiny battery can propel the car forwards at that speed in all conditions. It just means the motor-generator 1 (MG1) won't overrev, because if the internal combustion engine (ICE) is stopped, MG1 revs in reverse direction. The maximum speed at which ICE can be stopped seems to be 72 mph.
If you require lot of power at speeds below 72 mph, the ICE will spin up to provide that power.
Some older hybrid systems have even lower limits: my 2016 RAV4 hybrid seems to have a limit of 80 km/h (50 mph). For older Prius cars, the limit is 42 mph.
Do note that even at higher speeds when engine braking, even though the engine is spinning, no fuel is injected and the valves are adjusted for minimum resistance.
There's a variant of Prius, a plug-in hybrid that probably has even higher max speeds for EV mode. That's because the intention is that you can charge it from mains and drive using charged electricity.
Also, do note that for these non-plug-in hybrids, the charge in the battery will at most last for a mile if going at any significant speed. So, the max-EV speed is of academic interest only. In real driving, you will often have the ICE spinning.