higher mpg numbers

I'm working on trying to get my 98 4x2 4 cylinder Frontier as fuel efficient as possible. See my website at www.maxmpg.org. I also have a Yahoo forum of the same name. I'm obviously not into offroading and getting huge horsepower numbers, what I'm after is efficiency. I'm up to 28 mpg in the city. I generally don't speed much, I run my tires at 42 psi, and am modifying the engine for efficiency. My first big project is my radiator cooling fan. I removed the clutch driven fan and installed an electric one. It runs like a champ and boosted my mpg more than I thought it would. Previously I was getting only 23 to 27 mpg but now I'm getting consistent 27 to 28 in the city and over thirty miles per gallon on the highway. I'm currently building an aerodynamic cap that will slope from the top of the cab down to the tailgate. My next engine project will be to change from power to manual steering. Any ideas on how to accomplish this? I don't think Nissan ever made a manual sterring box for the Frontier. Does anyone know of another type that will bolt in? I also would like to put narrower, low rolling resistance tires. I like the Cooper tires mention in the previous post. I currently have 215/65 15 tires but I don't know which of the Cooper tires would be the best match for a narrower tire. I don't understand their sizing system. Any ideas? I want to keep the diameter as close as possible to what it is now so my mph and odometer readings will stay accurate. Thanks for any info.
John