So ,we've all heard it a million times, and I just want to remind ya all to check your tire pressure regularly. Case in point; My 2 prior tanks of all city driving were getting me about 20.5mpg (I do tend to drive my Juke like a motorcycle so I wasn't too surprized). When I filled up my last tank I checked my tire pressure (had not done this for about 4 months) and all tires were at about 30psi. Filled them to 36psi and with pretty much the same all city driving my mileage improved to 24mpg.
All I can say is "Duh!". Looks like I'll be replacing the air in my tires with nitrogen ASAP. Did this on my 2004 Frontier and I could go 8-10 months with absolutely no tire pressure loss.