2002 Mazda Tribute Heater kills mpg

I travelled a total 77 miles that day and I have a 15gal fuel tank which was full. I had the heater on during my travels (it was 25 degrees f out). At the end of the day I had half a tank of gas left. So 77miles/7.5 gal =something like 10.26666 mpg. If the heater were off I get between 20-25 mpg even in cold weather.

My gas gauge is reasonably accurate. This fluctuation in mpg only occurs when I run the heater. The vehicle burns extensively more fuel. I've tested this multiple times including an assumption that it had to do with my gas gauge, that assumption left me stranded on the side of the road. I understand it doesn't make sense to anyone that running a heater could cause a vehicle to burn double the fuel but it does. I preserve my gas mileage by leaving my heater off which, while effective, is very uncomfortable when it's below freezing.