Wow 37 mpg in summer? HELP

I'd suggest checking the tire pressures, and then if they are ok, check the 12 volt battery.

 

check the oil level too. the new tires might improve as they break in.

 

Answer the fuel economy questionnaire. . That's the first thing to do.

 

Batteries aren't measured in "awesome", they are measured in volts. Tires are measured many ways but the rolling resistance and PSI are the two most important.
Because it dropped so suddenly, my guess would be something related to your tire change. Right away with new tires it will drop. But because the wheels need to be re-aligned after, my gut feel says you need an alignment. Check your tires. Any uneven wear, even in the slightest? Do both your tires point straight with the steering wheel at 0-positon? If you take a long straight stick (like a good non-crowning 2x4) and put it next to the front and back tires, does it touch both tires on that side for the full length of the tire, or is one tire angled?

The other thing to check would be the brakes. Parking brake might have been mangled when the tires were changed and is now dragging. Could have busted a normal pad to drag. Are any of your wheels very hot after you go for a long drive?

 

Check the calibration of your tripmeter miles against either GPS or mile-markers on the road. If your tires are slightly larger diameter:

• indicated mph will be off - reading lower than true speed
• indicated MPG will be off - reading lower than true

For good measure, feel the tires after driving a bit and the hubs. You're looking for heat, the signature of energy loss.

Also, slack off the speed. You need to be running cruise control at 65-70 mph. Your photo obscured the average mph so it was likely to be 80 mph or better.

Bob Wilson