90 S10 2.8 V6 transmission help

I just swap my 5 speed manual transmission out for a used one because mine went out. The problem is that now when am in 5th gear the shift light come on, and the speed ohmmeter read right. But am also know getting 15 MPG wear I was getting 25 miles to MPG with the old transmission.

When your cruising down the road, what RPM are you cruising at?

i dont have a tachometer