YotaTech Forums - Misc questions about my new-to-me '00 PreRunner

I got my Prerunner about two weeks ago and have been searching the forum laboriously for answers to my many questions. I have found answers for most of them, which is a credit to the wealth of information on this site. A few remain, however:

1.) What sort of fuel economy should I expect? The best so far has been 19.5mpg with mixed driving biased toward highway and I don't drive aggressively. I expected it to be a lot lower than my '94's 27mpg, but I'm a little disappointed with below 20. I had hoped that maybe 21-22 would be possible. It has a clean K&N filter and no DTCs if that means anything. The plugs look kind of spent, so I'm installing new Denso 3194s today.

2.) Did all the PreRunners come with 31x10.5 tires? Are 225/75-15s an option? What about 30x9.5s? I checked the speedo against GPS and it's about 1.5-2mph slow at 65mph. I ask because the current tires are 31x10.5 Bridgestone Duelers that are nearly spent, so I need new ones anyway. The previous (original) owner said it's always had 31s and I'm willing to keep the size(they look good!) if that's the best option.

3.) What is the Denso part number for the front O2 sensor with plug? Is it 234-4162? All I could really find here was related to 4Runners.

4.) I plan on doing a complete trans fluid change and at least clean the pan magnets. Would Yamabond 4/Threebond 1194 be a suitable alternative to the FIPG used on the transmission pan?

5.) How important is replacing the trans filter(screen?)? Can it be cleaned instead, or is that a bad idea? I mean, Toyota doesn't even provide a replacement interval.