We're at about 15% now but let's be generous and use 20%. Neglecting the mounting angles and resulting reflection, that gives 640 watts collectible if you cover the whole car.
So you're collecting 3.2 miles of range per hour of peak sun. Note that the sun is not at peak elevation for 12 hours per day.
The middle of the country averages about 5 peak-sun-hours per day, so that means you're adding 15 miles of range per day of sitting in the sun.
It's not nothing, but it's very close to nothing. Useful to keep the car from self-discharging while it sits in long-term parking at the airport, perhaps. But it's not gonna provide meaningful range extension. The average EV has about 100-mile range, so if we assume a moving average of 33mph to make the math easy, you're moving for 3 hours, during which you collect enough power to actually move for 10 more miles. If you happen to be driving at noon in summer...