Why would your 98 Chevy S-10 vibrate only at speeds of 20-25 MPH but others speeds it does vibrate not no matter what gear your in?
Variety of possiblities. The first item I would check is the center support bearing. It is in about the middle of the truck and the drive shaft goes into it. The rubber begins to give out and the bearing is no longer supported well causing a vibration.
need front end alignment and wheels balanced, OR you could have a worn Ujoint that needs replacing! I have a 92 g20 with 350 the reason mine did was because the torque coverter went out . it viberates because when i would get to crusing speed the converter was stuck in geer, when i let off of the fuel or gave it fuel the viberation would stop.
I have a 1999 Chevy Malibu with a four cylinder. I drive a combination of city/highway including hwy speeds of 70-75mph and get a combined average of approximately 24-25mpg regularly. If driven at slower speeds while on mainly freeway you can get up to 28-29mpg but I have only done that acouple of times to see what I got.