I have a 90 300zx N/A 2+2. The engine is stock with the only mod is Maganflow Exhaust system and a Stillen Body kit. My Z revs at about 3700 rpms while going down the interstate crusin at 80. Is that normal ...? i only get about 15 mpg city and highway consistiently. My check engine light is not on. I run 91 octane gas and royal purple 10w30 motor oil with a recent tune up. What could be the problem of this? also after driving down the interstate for an 30 mins or so at 3700 rpms the temp guage goes up to about 3/4 the guage and stays there. is that normal also?