p0500 code on 97 chev

I have a 97 chevy 2500 2wd with a 4l80E tranny. Well, i got done with work, was driving home and the check engine light came on. :uhoh: Anyhow, i went to the shop teachers house and got him to open the shop and use the schools computer and found it out to be code p0500, or something like that. Is was low vacume ( under 20 whatever it is) above 140 degrees and between 1400-4400 RPM. Well, this is the first time it threw this code. This truck has always has gas milage problems. Chevy says it should get 18, and the best we have now is 10. I have changed both upstream oxygen sensors, took a fuel pressure test, and found it to be low (right after i changed the filter), so i changed the pump and the relay), i now have 65 PSI of pressure. Before all of this i had 6 MPG, so it is coming up. Where should i check for vacume? How much vacume should this engine have? In the schools computer i am planning on having that hooked up, and having the mechanical vacume gauge hooked up, and see if they are on of the same. If they are different, it is the MAP sensor right?

My book says a P0500 is a vehicle speed sensor code. Is your speedometer correct?

yeah, turn out the MAP sensor was bad. Vacume readings and the computer reading were different, have better milage and more power now.