97 3.4 AT. I get ~18 mpg average.

Watch your tire pressure. Low pressure creates a good bit of drag and can really pull down mileage. Run the pressure as high as you can stand it.

My mileage doesn't vary much normally. Be it highway driving, town driving, or pulling a trailer.

BUT... locking out overdrive once thoroughly killed gas mileage. Over around 75 mph drops my mileage. Lots of full throttle romping hurts my mileage.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)