Well truthfully you should be getting better fuel economy in this heat. The added heat helps with fuel vaporization and up to a point it should help with fuel economy all other things being equal. I dont think I have EVER gotten that poor of fuel economy out of my GP. Even with mostly city driving I think the worse Ive ever seen out of it was 19 and that was with some spirited driving too.

I would say you have other things going on but its not heat related.

Now, you also made a statement that I hear a lot when I talk to people about fuel economy. "I swear my mpg is sitting...." or "I think I get..." or "Somewhere around..."

If you are swearing and cannot say factually, then that tells me you are guessing to some extent. If you go off and try and troubleshoot a fuel economy issue with guesstimations, you will run yourself ragged and chase your tail. You really need documentation.

I keep extensive fuel logs on my vehicles. I write down both the trip odometer and regular odometer readings on every fuel receipt then manually calculate the mileage for that tank and then keep all that in a fuel log. When I fill up, I fill that thing until it cannot take not one more drop of fuel. This way I get consistent readings. Filling it up until the handle clicks off, well you're just going to be all over the place.

The best thing for you to do is to fill that tank up until it runneth over. Then drive until you feel the need to refuel. Fill it up the same way again and then manually calculate your mileage. Then you need to do that for a few tanks. Preferably you would do it on every tank to have a running history and long term analysis of what your fuel economy is doing over the spring, summer, fall and winter months.

But after you do it for at least a couple of tanks, then let us know what you are getting for sure. But I assure you that the heat is not the reason your economy is bad, that actually helps it. Its also the reason most vehicles come from the factory with a 195* thermostat in them. It promotes fuel heating and that promotes better fuel vaporization which in turn produces a better burn and thus better economy.