I installed a block heater on my TCH just to see if it would help any on the gas mileage. I did find I could get the car in the EV mode earlier. That is if the air temp was about 50 degrees. This only helped for about a mile from the house were we live.

I worried on those 30 to 35 degree days why the engine temperature was only 85 to 100 degrees at crank up. (reading from the scan gauge)

I decided to call a North Dakota Toyota dealer. I found thats normal for a very cold day even if the block heater had been plugged in for hours. I also was told the 'new' block heaters don't work unless the outside temperature is at zero or below. Toyota found out the block heaters were fooling the cars computer. It was causing the engine not to fully warm up the catalytic converter.

I believe block heaters are designed for the cold weather states. I doubt they are needed as much with the thin full synthetic oils like, 0W-20.