My highway mileage starts dropping at rpms of 2000 or higher, which means the faster I go over 70, the worse my mileage is. Hot humid air cuts it down too.

 

I'll guess that your city driving is pushing your average down. To eliminate differences in shifting styles, the next time you're out on open interstate and all shifted up to cruising speed, reset the mpg counter. If after maybe 10 miles of uninterrupted cruising you're getting 29+, all is well. If not, hmmm, dunno. It sounds like our two cars are configured very similarly.

Last time I filled up, I'd been zipping around in town for a while and was down to 22 mpg. But that mostly was about getting out of the way of other traffic, idling at stoplights, and running the AC full blast. I will admit to a pretty easy commute: 5 miles of widely space stoplights followed by 15 miles of open 75mph 4-lane. Not a lot of gas-wasting minivan-dodging going on there. Your daily driving may not be quite that easy on the gas.