Are you on stock wheels?
It is cold ambient air temp. The last trip was Seattle to Great Falls, MT. 10 hour drive. If I was going to go 15mph under the speed limit it would be a 12 hour drive .You were able to get 40 mpg on one tank so you're not too far off from the rated 43 mpg. Good suggestions from others regarding tire pressure and speed. I would also just get a new engine air filter unless you know it was changed recently. Change the cabin filter while you're at it, won't affect mpg, both can be bought on ebay as a combo pack for $12. Little changes here and there will add up to improved mpg.
I didn't mean drive that speed all the time. I meant try driving 60 MPH for example on some highway in the right lane and watch your fuel economy gauges to see what your car is showing you're getting. This will give you a baseline comparison to other speeds and factors in your normal driving. I drove every hybrid I've ever tried at 60 MPH on the highway with cruise on and use that to compare them. It's the closest I get to lab-specific conditions across each vehicle drive.It is cold ambient air temp. The last trip was Seattle to Great Falls, MT. 10 hour drive. If I was going to go 15mph under the speed limit it would be a 12 hour drive .